JL · AP MICROECONOMICS

Unit 4（下）· Monopolistic Competition and Oligopoly

垄断竞争与寡头 — 产品差异化、广告、长期零利润与产能过剩；寡头、合谋与卡特尔、收益矩阵、优势策略、纳什均衡、囚徒困境 · 对应 CED 4.4–4.5 · Unit 4 选择题占比约 15–22%（上下册合计）

上下册分工上册：市场结构总览、垄断与价格歧视（4.1–4.3）。→ 打开 Unit 4（上）Monopoly and Price Discrimination（与本文件放在同一文件夹）
下册（本册）：介于完全竞争和垄断之间的两种结构——垄断竞争（4.4）与寡头（4.5），以及分析寡头的工具：博弈论。

从上册带过来的工具

需求向下倾斜的企业：MR 在需求下方（同一纵截距、斜率加倍），多卖一个要降低所有单位的价格。
任何企业都在 MR = MC 处定产量，价格从需求曲线上读，利润 = (P − ATC) × Q。
P > MC → 配置无效率，有无谓损失；不在 ATC 最低点生产 → 生产无效率。
Unit 3 的进入与退出：有经济利润就有企业进入，有亏损就有企业退出，直到经济利润为 0——只要没有进入壁垒。

本册学习目标 LEARNING OBJECTIVES

说出垄断竞争的特征，解释产品差异化与广告的作用 · Describe monopolistic competition, product differentiation, and advertising
画出垄断竞争企业的短期盈利、亏损图，以及长期均衡（需求与 ATC 相切） · Graph short-run profit/loss and long-run equilibrium
解释长期零利润、产能过剩与 P > MC，并与完全竞争、垄断比较 · Explain excess capacity and inefficiency
说出寡头的特征，解释合谋、卡特尔和作弊的动机 · Explain oligopoly, collusion, cartels, and the incentive to cheat
读收益矩阵，找出优势策略和纳什均衡，识别囚徒困境 · Find dominant strategies and Nash equilibria in a payoff matrix
收益改变后重画矩阵，计算改变优势策略所需的激励，算出合并后的最大总利润 · Redraw matrices and calculate the incentive needed to change a dominant strategy

Term 术语	Meaning 含义	中文
Monopolistic Competition	Many firms, differentiated products, free entry and exit	垄断竞争
Product Differentiation	Making a product different from rivals' (style, location, quality, brand)	产品差异化
Nonprice Competition	Competing through advertising, design, service rather than price	非价格竞争
Excess Capacity	Output below the quantity that minimizes ATC	产能过剩
Oligopoly / Duopoly	A few interdependent firms; two firms	寡头 / 双头垄断
Interdependence	Each firm's profit depends on rivals' choices	相互依存
Collusion / Cartel	Firms coordinate to act like a monopoly; a formal collusive agreement	合谋 / 卡特尔
Game / Strategy / Payoff	Interaction among players; a complete plan of actions; the result for each player	博弈 / 策略 / 收益
Payoff Matrix (Normal Form)	Table showing the payoffs for each combination of strategies	收益矩阵
Dominant Strategy	A strategy that is best no matter what the other player does	优势策略
Nash Equilibrium	No player can gain by changing strategy alone, given the others' strategies	纳什均衡
Prisoner's Dilemma	Each player's dominant strategy leads to an outcome worse for both	囚徒困境

考试怎么考选择题：垄断竞争的长期均衡条件（P = ATC、MR = MC、P > MC、产能过剩）几乎每年都考；广告让需求更缺乏弹性；寡头的“相互依存”；卡特尔为什么不稳定；读收益矩阵找优势策略和纳什均衡。 FRQ：近年每套题几乎都有一道收益矩阵短题（5–6 分）：某方的最优反应、有没有优势策略、纳什均衡、收益改变后重画矩阵、需要多少激励才能改变优势策略、两家合并后的最大总利润。

4.4Monopolistic Competition 垄断竞争

大学城的一条街上有十几家炸鸡店：有的主打韩式甜辣，有的主打脆皮，有的离宿舍最近，有的能送外卖。每家都能自己定价，但如果比别人贵太多，顾客就去隔壁了；生意好的话，很快就会有新店开出来。这就是垄断竞争——名字里既有“垄断”（自己的产品独一份），也有“竞争”（很多相似的对手、进出自由）。

Characteristics 特征与产品差异化

定义与特征（CED PRD-3.B.10） Monopolistic competition：

企业很多：每家只占市场的一小部分，不会去盯着某个对手的反应（这一点和寡头不同）。
产品有差异（differentiated products）：相似但不相同，所以每家面对向下倾斜的需求，有一点定价能力。
进入和退出自由（low barriers to entry）：所以长期经济利润会被推到 0（这一点和垄断不同）。
常用广告和品牌来突出自己的不同——即非价格竞争（nonprice competition）。

产品差异化的几种方式

🍗

风格或种类：韩式炸鸡、美式炸鸡、无骨炸鸡；同样是洗发水，有去屑的、控油的、香味不同的。

📍

位置：宿舍楼下的便利店可以比 1 公里外的贵一点——顾客为“近”付钱。

⭐

质量：用料更好、服务更周到的理发店，收费更高也有人去。

📣

品牌与广告：即使产品本身差不多，熟悉的牌子也让人更放心（广告也是一种“质量信号”）。

三家企业现在都卖 $10、卖 4 个。把价格提到 $12：垄断（没有相近替代品）只少卖 0.5 个；垄断竞争企业（附近有很多相似的店）少卖 2 个；完全竞争企业一个也卖不出去。垄断竞争企业的需求向下倾斜（有一点定价能力），但比垄断平坦得多——替代品越多，需求越有弹性。

企业怎么决策：和垄断一样需求向下倾斜 → MR 在需求下方（多卖要降价）。产量定在 MR = MC，价格从需求曲线上读，利润 = (P − ATC) × q。区别只在于：垄断的需求就是整个市场的需求；垄断竞争企业的需求只是它自己那一份，替代品多，所以更有弹性，而且会随着对手的进出而移动。

易错 TRAP

垄断竞争企业的需求向下倾斜，是因为产品有差异，不是因为企业多、也不是因为进入容易。
它不是 price taker：MR < P。只有完全竞争企业才有 MR = P。

Advertising 广告与非价格竞争

完全竞争的农民不打广告，垄断竞争的企业却天天打。广告的目的有两个：让需求增加（更多人想买这一家），以及让需求更缺乏弹性（顾客更忠诚，涨一点价也不走）。

一次成功的广告：顾客更多（D₁ → D₂ 右移），而且更认这个牌子，涨价时流失的人更少（需求更陡、更缺乏弹性）。广告费是一笔固定开支（不随卖多少变化）：ATC 上移到 ATC₂，MC 不变。广告值不值得做，要看增加的收益能不能超过广告成本。

易错 TRAP · 广告让需求“更缺乏弹性” 选择题常问 “A monopolistically competitive firm advertises in order to…” → 答案是 make demand less elastic（或 increase demand / shift demand right）。“make demand more elastic”“shift demand left”“reduce barriers to entry”都是错的。

WORKED EXAMPLE 1火锅店该投几条短视频广告？原创

A hot-pot restaurant in a monopolistically competitive market is deciding how many promotional videos to buy on a short-video app. The table shows the total benefit (extra profit before paying for the ads) and the total cost of the videos.

Number of videos	1	2	3	4	5	6
Total benefit ($)	1,500	2,700	3,600	4,200	4,500	4,600
Total cost ($)	400	900	1,500	2,200	3,000	3,900

(a) Calculate the total net benefit of buying 2 videos. (b) Calculate the marginal net benefit of the 3rd video. (c) What is the optimal number of videos? Explain using marginal analysis. (d) A holiday season raises the marginal benefit of every video by $200. What is the new optimal number? (e) Why does a firm in this market advertise, while a wheat farmer does not?

第几条	1	2	3	4	5	6
MB = ΔTB	1,500	1,200	900	600	300	100
MC = ΔTC	400	500	600	700	800	900
净收益 TB − TC	1,100	1,800	2,100	2,000	1,500	700

(a) 总净收益 = 2,700 − 900 = $1,800。

(b) 第 3 条的边际净收益 = MB − MC = (3,600 − 2,700) − (1,500 − 900) = 900 − 600 = $300。

(c) 第 3 条 MB $900 > MC $600，值得投；第 4 条 MB $600 < MC $700，不值得 → 3 条（净收益最大 $2,100）。

(d) MB 变成 1,700、1,400、1,100、800、500、300：第 4 条 800 > 700 值得，第 5 条 500 < 800 不值得 → 4 条。

(e) 小麦是同质产品，农民按市场价想卖多少卖多少，打广告不能让人只买他家的麦子。垄断竞争企业的产品有差异，广告能让更多人想买这一家，并让顾客更忠诚（需求更缺乏弹性）。

每多投一条视频：蓝柱 = 多带来的利润（MB），红柱 = 多花的钱（MC）。前 3 条 MB > MC，第 4 条起 MB < MC——投 3 条最好。

练习 Practice 4.4（一）· 特征、需求与广告

以下为历年 AP 真题（带图、表和收益矩阵的题按原题重绘），点选项即时判分并显示解析。

4.4-12010 AP Micro Q53

An industry consists of 100 small firms, and the largest firm accounts for only 2 percent of sales. Brand names are considered a signal of quality. The industry described is best classified as

monopoly
perfectly competitive
monopolistically competitive
oligopolistic
monopsonistic

100 家小企业、最大的只占 2%（企业很多），品牌被当作质量信号（产品有差异） → 垄断竞争。

4.4-22013 AP Micro Q51

Over the past 5 years, 50 new restaurants have opened and 30 have closed in the city of Zuni. Currently there are 110 restaurants operating in the city. Which of the following best represents the market structure, barriers to entry, and economic profits in the long run?

Market Structure: Perfect competition；Barriers to Entry: Low；Long-Run Economic Profits: Negative
Market Structure: Perfect competition；Barriers to Entry: High；Long-Run Economic Profits: Positive
Market Structure: Monopolistic competition；Barriers to Entry: Low；Long-Run Economic Profits: Zero
Market Structure: Monopolistic competition；Barriers to Entry: High；Long-Run Economic Profits: Positive
Market Structure: Monopolistic competition；Barriers to Entry: High；Long-Run Economic Profits: Zero

餐馆很多、风格各异，5 年里开了 50 家、关了 30 家（进出自由） → 垄断竞争、进入壁垒低、长期经济利润为 0。

4.4-32014 AP Micro 练习卷 Q24

Which of the following statements correctly identifies a difference between perfect competition and monopolistic competition?

In perfect competition there are no barriers to entry, but there are strong barriers in monopolistic competition.
In perfect competition there are many firms, but in monopolistic competition there are only a few firms.
In perfect competition the firms all sell products that are exactly the same, but in monopolistic competition each firm sells a slightly differentiated product.
In perfect competition firms maximize profit by selling the quantity where marginal revenue equals marginal cost, but in monopolistic competition firms maximize profit by selling the quantity where marginal revenue exceeds marginal cost.
In perfect competition there are few consumers, but in monopolistic competition there are many consumers.

两者都有很多企业、进出自由、都在 MR = MC 处生产；区别在产品：完全竞争完全相同，垄断竞争略有差异。

4.4-42015 AP Micro 练习卷 Q24

All of the following characterize both perfectly competitive and monopolistically competitive markets EXCEPT:

Price is equal to average revenue.
Individual firms produce output where marginal cost equals marginal revenue.
Firms can affect the selling price of their product.
The market has a large number of firms.
Firms can easily enter or exit the market.

完全竞争企业是 price taker，不能影响价格；垄断竞争企业可以。(A) 两者都 P = AR；(B)(D)(E) 都是共同特征。

4.4-52016 AP Micro 练习卷 Q11

The demand curve for a monopolistically competitive firm is downward sloping because

there are a large number of firms
the product is produced by using scarce resources
the products produced by different firms are not identical
it is easy for firms to enter or exit the market
the marginal cost rises as output produced increases

需求向下倾斜是因为产品不完全相同：涨价时只流失一部分顾客，喜欢它的人还会买。企业多、进入容易反而让需求更有弹性。

4.4-62010 AP Micro Q41

A well-known fast-food franchise substantially increases the price of its hamburgers, and loses only some of its customers. Which of the following best explains why the franchise has not lost all of its customers?

Its hamburgers are a perfect substitute for other types of fast food.
Its hamburgers are differentiated.
The demand for its hamburgers is perfectly elastic.
The other competitive fast-food restaurants decrease the price for their hamburgers.
The barriers to entry are very low for entrepreneurs trying to enter the fast-food business.

涨价只失去一部分顾客 → 需求不是完全弹性 → 因为汉堡有差异（口味、品牌），顾客不把别家当作完全替代品。

4.4-72019 AP Micro 练习卷 Q13

In monopolistic competition, an individual firm’s market power stems from which of the following?

Economies of scale in production
The production of a differentiated product
The absence of perfectly competitive firms that produce similar products
Barriers to entry allowing firms to earn economic profit
The maintenance of excess capacity to meet unexpected increase in demand

垄断竞争企业的市场势力来自产品差异化；它没有显著的规模经济，也没有进入壁垒。

4.4-82013 AP Micro Q56

A monopolistically competitive firm’s demand curve will be highly elastic if which of the following exists?

A high degree of product differentiation
A highly elastic supply curve for the firm
High barriers to entry in this industry
A high degree of product substitutability
A small number of competitors

替代品越多、越相似（product substitutability 高），顾客越容易转走 → 需求越有弹性。(A) 差异化程度高反而让需求更缺乏弹性。

4.4-92012 AP Micro Q57

A monopolistically competitive firm advertises in order to

shift the demand curve for its product to the left
make the demand for its product less price elastic
make its product more similar to its competitors’
increase its positive externalities
reduce the industry’s barriers to entry

广告让顾客更忠于这个牌子，涨价时流失更少 → 需求更缺乏弹性（也会让需求右移）。(A) 方向反了；(C) 广告是要突出不同。

4.4-102018 AP Micro 练习卷 Q41

In monopolistic competition, a goal of advertising is to

reduce a firm’s short-run average total cost
minimize a firm’s long-run average total cost
make a firm’s demand curve more elastic
make a firm’s demand curve less elastic
shift a firm’s demand curve to the left

广告的目标：让企业的需求更缺乏弹性（并增加需求）。广告费会提高 ATC，而不是降低。

4.4-112021 AP Micro 国际卷 Q33

Which of the following is true of the marginal revenue curve for a monopolistically competitive firm?

It is the same as the demand curve because there are many firms in the market.
It is the same as the demand curve because marginal revenue is always equal to price.
It is upward sloping because total revenue increases as the quantity sold increases.
It lies below the demand curve because the firm must lower its price for all units in order to increase sales.
It lies below the demand curve because marginal revenue increases when price decreases.

要多卖一个就得降低所有单位的价格 → MR 在需求下方。(A)(B) 是完全竞争；(C) MR 向下倾斜；(E) 原因说错了。

4.4-122015 AP Micro 练习卷 Q59

Which of the following about the relationship between marginal revenue (MR) and price (P) under monopolistic competition and perfect competition is correct?

Monopolistic Competition: MR>P；Perfect Competition: MR=P
Monopolistic Competition: MR<P；Perfect Competition: MR=P
Monopolistic Competition: MR decreases when P increases.；Perfect Competition: MR increases when P decreases.
Monopolistic Competition: MR=P；Perfect Competition: MR=P
Monopolistic Competition: MR=P；Perfect Competition: MR<P

垄断竞争企业需求向下倾斜 → MR < P；完全竞争企业是价格接受者 → MR = P。

Short Run 短期：可能盈利，也可能亏损

短期内店数来不及变化，一家店可能赚钱、可能亏钱，也可能刚好不赚不亏——取决于需求在 MR = MC 的产量处高于还是低于 ATC。

短期盈利：需求 P = 34 − 3q 时，① MR = MC 定产量 q = 4；② 往上读需求，P = $22；③ 此时 ATC = $15，经济利润 = (22 − 15) × 4 = 28（绿色矩形）。画法和垄断完全一样。

短期亏损：需求弱到 P = 18 − 3q，MR = MC 在 q = 2，P = $12，ATC = $22，每份亏 $10，共亏 20（红色矩形）。但 P = $12 > AVC = $4，营业能收回一部分固定成本（亏 20 < 停业亏掉的固定成本 36），所以短期继续营业——停产规则和 Unit 3 一样。

短期三条规则（与 Unit 3、上册相同）

产量：MR = MC；价格：读需求。
P > ATC 盈利，P = ATC 正常利润，P < ATC 亏损。
亏损时：P ≥ AVC 继续营业；P < AVC 停业（产量为 0）。

WORKED EXAMPLE 2大学城的炸鸡店原创

A fried-chicken shop near a university is one of many shops selling slightly different fried chicken. It faces the demand curve P = 34 − 3q, where q is hundreds of boxes per week. Its marginal cost is MC = 2 + 2q and its average total cost is ATC = 2 + q + 36/q.

(a) Write the equation of the shop's MR curve. (b) Find the profit-maximizing quantity and price. (c) Calculate the shop's economic profit. (d) What will happen in this market in the long run? (e) In the long run the shop's demand becomes P = 26 − 3q. Find the long-run quantity, price, and economic profit.

(a) 同一纵截距、斜率加倍：MR = 34 − 6q。

(b) MR = MC：34 − 6q = 2 + 2q → q = 4（400 盒）；价格读需求：P = 34 − 12 = $22。

(c) ATC = 2 + 4 + 9 = $15，利润 = (22 − 15) × 4 = 28（百美元）= $2,800 每周。

(d) 有经济利润、进入容易 → 新的炸鸡店进入，每家店的顾客变少，需求（和 MR）左移，直到经济利润为 0。

(e) MR = 26 − 6q = 2 + 2q → q = 3，P = 26 − 9 = $17；ATC = 2 + 3 + 12 = $17 = P → 经济利润 = 0。需求正好在 q = 3 处与 ATC 相切。

练习 Practice 4.4（二）· 短期决策

以下为历年 AP 真题（带图、表和收益矩阵的题按原题重绘），点选项即时判分并显示解析。

4.4-132022 AP Micro 国际卷 Q32

Which of the following is true for a monopolistically competitive firm that is in short-run equilibrium and earning a positive economic profit?

Accounting profits are less than economic profits.
Marginal cost is greater than price.
Marginal revenue is less than price.
Average total cost is greater than price.
Marginal revenue is less than marginal cost.

需求向下倾斜 → 任何产量下（第一单位之后）MR < P。(A) 会计利润 ≥ 经济利润；(B)(D)(E) 与“利润最大且为正”矛盾。

4.4-142018 AP Micro 练习卷 Q38

Assume that a monopolistically competitive firm is currently maximizing profit with an output of 100 units and a price of $50. Which of the following is true?

Total revenue is maximized when the firm produces 100 units of output.
Marginal revenue equals $50 when the firm produces 100 units of output.
Marginal cost is greater than marginal revenue when the firm produces 150 units of output.
Marginal cost is minimized when the firm produces 100 units of output.
Average total cost is minimized when the firm produces 100 units of output.

100 个单位时 MR = MC。再往后 MR 继续下降、MC 在 MR 上方 → 在 150 个单位时 MC > MR。(A) TR 最大要 MR = 0，而这里 MR = MC > 0；(B) MR < P = $50。

4.4-152024 AP Micro Q44

Gatafab produces shoes in a monopolistically competitive market. Currently, Gatafab is producing 100 pairs of shoes. Its marginal cost is $15, price is $20, marginal revenue is $15, and average total cost is $17. What is Gatafab's total economic profit?

$3
$5
$300
$500
$2,000

利润 = (P − ATC) × Q = (20 − 17) × 100 = $300。MR = MC = $15 说明它已在利润最大的产量。

4.4-162025 AP Micro 国际卷 Q37

A firm in a monopolistically competitive market is producing the profit-maximizing quantity at 60 units of output at a price of $12 per unit. What is the firm’s economic profit if the firm’s average variable cost is $8, its marginal cost is $7, and its average fixed cost is $2 at 60 units of output?

−$300
$120
$180
$240
$300

ATC = AVC + AFC = 8 + 2 = $10，利润 = (12 − 10) × 60 = $120。MC = $7 是干扰数据。

4.4-172023 AP Micro 国际卷 Q23

Greencom is currently producing 20 units of output. At this level of output, Greencom’s marginal revenue is $6, marginal cost is $6, and average total cost is $9. If Greencom operates in a monopolistically competitive market and sells all 20 units of output for $10 per unit, it can be concluded that

Greencom would earn more profit by decreasing output
Greencom would earn more profit by increasing output
Greencom is earning zero economic profit
Greencom is earning negative economic profit
Greencom is earning positive economic profit

MR = MC = $6，已是利润最大产量（A、B 错）；P = $10 > ATC = $9 → 经济利润 = (10 − 9) × 20 = $20 > 0 → 正的经济利润。

4.4-182023 AP Micro 国际卷 Q26

The table provided shows price and cost data for Howell’s Toy Hoops, a typical profit-maximizing firm that sells its toys in a monopolistically competitive market. At the profit-maximizing quantity, the economic profit for Howell’s Toy Hoops is

Quantity	Price ($)	Total Cost ($)
1	10	11
2	8	14
3	7	18
4	5	25
5	3	34

−$19
−$5
−$1
$2
$3

TR = 10、16、21、20、15；利润 = TR − TC = −1、2、3、−5、−19 → 最大利润在 Q = 3，为 $3。

4.4-192017 AP Micro 练习卷 Q40

Which of the following must be true if a profit-maximizing monopolistically competitive firm continues to operate in the short run while incurring a loss?

Marginal revenue equals marginal cost and price is greater than average total cost.
Marginal revenue equals marginal cost and price is greater than average variable cost.
Marginal revenue equals marginal cost and price is less than average variable cost.
Price equals both marginal cost and marginal revenue.
Price is less than marginal revenue, but greater than average variable cost.

亏损还继续营业 → 价格至少能覆盖 AVC。所以：MR = MC（利润最大/亏损最小）且 P > AVC。(A) P > ATC 就不是亏损了。

4.4-202017 AP Micro 练习卷 Q27

A monopolistically competitive firm is currently producing the profit-maximizing level of output. If the price of a variable input increases, which of the following will occur?

The firm will increase its output to increase its revenue.
The firm will increase the price of its output by the same amount.
The firm’s average total cost and marginal cost curves will shift upward.
The firm’s average fixed cost will decrease as it decreases production.
The firm’s fixed cost will increase, but its output level will be unaffected.

可变投入变贵 → 可变成本上升 → AVC、ATC、MC 都上移（固定成本不变）。企业会减产、提价，但提价幅度不一定等于成本上升幅度。

4.4-212017 AP Micro 练习卷 Q47

Questions 47-48 refer to the following graph, which shows the cost and revenue curves for a profit-maximizing monopolistically competitive firm.

按原题重绘

What is the profit-maximizing price and quantity?

Price: P₁；Quantity: Q₁
Price: P₂；Quantity: Q₄
Price: P₃；Quantity: Q₃
Price: P₄；Quantity: Q₂
Price: P₅；Quantity: Q₁

MR = MC 在 Q₁（MR 与 MC 交于 P₁ 高度）；从 Q₁ 往上到需求读价格 → P₅、Q₁。P₁ 只是交点的高度。

4.4-222019 AP Micro 练习卷 Q39

Questions 39-40 refer to the cost and revenue conditions of a monopolistically competitive firm shown in the graph below. MC = marginal cost, ATC = average total cost, AVC = average variable cost, and MR = marginal revenue.

按原题重绘

The firm’s profit-maximizing output in the short run is

zero, because P < AVC
Q₁, because MR = MC
Q₂, because P = MC
Q₃, because MC = ATC
impossible to determine

在 MR = MC 的 Q₁ 处，需求给出的价格（在 P₂ 与 P₃ 之间）低于 AVC（= P₃） → 营业连可变成本都收不回 → 产量为 0（停业）。

4.4-232019 AP Micro 练习卷 Q40

按原题重绘

Which of the following will the firm do in the long run if market conditions do not change?

It will increase output to Q₂ and lower price to P₂ to minimize losses.
It will increase output to Q₃ and raise price to P₄ to earn zero economic profit.
It will produce Q₁ and set price equal to marginal revenue.
It will exit the industry.
It will build a larger plant to achieve decreasing returns to scale.

ATC 在每个产量都高于需求 → 无论生产多少都亏损，市场条件不变就会一直亏 → 长期退出行业。

Long Run 长期：进入与退出把利润推到 0

垄断竞争的长期调整和完全竞争一样靠进入与退出；不同的是，完全竞争是“市场价格”在变，而垄断竞争是“每家店面对的需求”在移动。点按钮一步一步看：

① 短期：有利润：需求 D₁ 时，q = 4、P = $22 > ATC = $15，有经济利润。开店很容易（进入壁垒低），利润会把新店吸引进来。

② 新店进入：附近的炸鸡店越来越多，顾客被分走，每家店面对的需求和 MR 一起左移（D₁ → D₂）；替代品更多，需求通常也变得更有弹性。

③ 长期：利润归零：进入一直持续到需求 D₂ 正好与 ATC 相切：q = 3，P = ATC = $17，经济利润 = 0，再没有新店想进来。注意 MR = MC = $8 仍然成立，P = $17 > MC。

如果一开始是亏损，过程反过来：

① 短期：亏损：需求 D₀ 时，q = 2、P = $12 < ATC = $22，每家都在亏损（但 P > AVC，短期继续营业）。

② 有店退出：长期亏损的店关门，它的顾客转到留下的店，每家店的需求和 MR 右移（D₀ → D₂）。

③ 长期：利润归零：退出一直持续到需求与 ATC 相切：q = 3，P = ATC = $17，经济利润 = 0。不管从盈利还是亏损出发，长期都回到同一个结果。

长期均衡的条件（选择题必考） MR = MC　·　P = ATC（需求与 ATC 相切）　·　P > MC

经济利润 = 0（只赚正常利润），这一点和完全竞争相同。
相切点在 ATC 最低点的左边（向下倾斜的需求只能在 ATC 下降的那一段与它相切），所以 MC < ATC，产量小于 ATC 最低的产量。
P > MR = MC，所以 P > MC，这一点和垄断相同。

易错 TRAP

长期利润为 0 不等于“P = MC”。垄断竞争长期是 P = ATC，不是 P = MC。
长期均衡的相切点不是 ATC 最低点，也不是 MC 与 ATC 的交点（选项里写“MC = ATC”的是完全竞争）。
“短期还是长期？”看利润：图上 P > ATC 或 P < ATC 都只能是短期；P = ATC（相切）才是长期。

LAB 1开店还是关店？ · Long-Run Adjustment

大学城的炸鸡店，每家成本都是 MC = 2 + 2q，ATC = 2 + q + 36/q。店越多，每家分到的顾客越少：每家面对的需求是 P = a − 3q，其中 a = 46 − 2 × 店数。拖动店数，看一家店的利润；或者点“让市场自己调整”，看进入和退出把利润推向 0。

店数 N = 6

Efficiency 产能过剩与 P > MC

长期均衡的两个“没效率”：① 产能过剩——在 q = 3 生产，而 ATC 最低的产量是 q = 6（ATC 最低 $14，现在却是 $17），没有达到生产效率；② P > MC——最后一份炸鸡顾客愿付 $17，多做一份只要 $8，没有达到配置效率。配置有效的产量在 D = MC 处（q = 4.8），红色三角形是 DWL = ½ × 9 × 1.8 = 8.1。

产能过剩 EXCESS CAPACITY（CED PRD-3.B.10）垄断竞争企业长期的产量小于使 ATC 最低的产量，两者之差就是 excess capacity——企业本可以多生产、把平均成本降下来。所以它没有生产效率（不在 min ATC 生产），也没有配置效率（P > MC）。

同样的成本曲线。左：完全竞争企业的需求是水平线，长期在 ATC 最低点生产（P = MC = min ATC = $14，q = 6）。右：垄断竞争企业的需求向下倾斜，只能在 ATC 最低点左边与 ATC 相切（P = ATC = $17，q = 3）——价格更高、产量更少，两者长期经济利润都是 0。

长期均衡	完全竞争	垄断竞争	垄断（有进入壁垒）
企业的需求	水平（P = MR）	向下倾斜、较平	市场需求，向下倾斜
产量规则	MR = MC	MR = MC	MR = MC
P 与 ATC	P = ATC	P = ATC（相切）	通常 P > ATC
经济利润	0	0	可以 > 0
P 与 MC	P = MC	P > MC	P > MC
是否在 min ATC 生产	是	否（产能过剩）	一般否
配置效率 / 生产效率	有 / 有	无 / 无	无 / 无

WORKED EXAMPLE 3炸鸡店的长期均衡：浪费了多少？原创

Continue with the fried-chicken shop in long-run equilibrium (P = 26 − 3q, MC = 2 + 2q, ATC = 2 + q + 36/q, q = 3, P = $17). (a) At what quantity is ATC minimized, and what is the minimum ATC? (b) How large is the shop's excess capacity? (c) Compare price with marginal cost. (d) Find the allocatively efficient quantity for this shop's demand, the deadweight loss, and the consumer surplus at the long-run equilibrium. (e) If this were a perfectly competitive industry with the same costs, what would the long-run price be?

(a) MC 穿过 ATC 的最低点：2 + 2q = 2 + q + 36/q → q² = 36 → q = 6，ATC = 2 + 6 + 6 = $14。

(b) 产能过剩 = 6 − 3 = 3（300 盒）：店面和炸锅能用更低的平均成本做更多，却只做了一半。

(c) MC(3) = 2 + 6 = $8，P = $17 > MC = $8：还有顾客愿意付高于 $8 的价钱，却没被服务。

(d) P = MC：26 − 3q = 2 + 2q → q = 4.8，P = $11.6。DWL = ½ × (17 − 8) × (4.8 − 3) = 8.1（百美元）。消费者剩余 = 价格 $17 以上、需求以下的三角形 = ½ × (26 − 17) × 3 = 13.5（百美元）；经济利润为 0。

(e) 完全竞争长期 P = min ATC = $14，比垄断竞争的 $17 低。

多样性的代价：每条街都有几家风格不同的炸鸡店，顾客走几步就能买到喜欢的口味——这份“选择多、离得近”的好处，正是用更高的平均成本换来的。经济学家对“垄断竞争到底浪费多少”并没有统一的结论。

WORKED EXAMPLE 4用字母读长期均衡图（AP 选择题考法）原创

The graph shows a monopolistically competitive firm in long-run equilibrium. Using the labels, identify (a) the profit-maximizing quantity and price, (b) the firm's economic profit, (c) the amount of excess capacity, (d) the allocatively efficient quantity and the area of deadweight loss, and (e) the price that would prevail if the firm's demand were perfectly elastic in long-run equilibrium.

一家处于长期均衡的垄断竞争企业：a 是需求与 ATC 的切点，b 是 MR = MC，c 是需求与 MC 的交点，d 是 ATC 最低点。

(a) MR = MC 在 b 点 → 产量 Q₁；往上到需求的 a 点 → 价格 P₃（不是 P₁）。

(b) 在 Q₁ 处，需求与 ATC 相切于 a 点：P = ATC = P₃ → 经济利润 = 0（正常利润）。

(c) ATC 最低点 d 的产量是 Q₃ → 产能过剩 = Q₃ − Q₁。

(d) 配置有效要求 P = MC → 需求与 MC 的交点 c，产量 Q₂；DWL = 三角形 abc。

(e) 需求完全有弹性就是完全竞争：长期在 ATC 最低点生产，价格 = P₂，产量 Q₃。

口诀：产量看 MR = MC，价格看需求，长期利润看“相切”，产能过剩看 ATC 最低点，DWL 看 D 与 MC 的交点。

AP 选择题的常见说法 “Monopolistically competitive firms are inefficient because…” → price is greater than marginal cost（配置）或 they do not produce at minimum ATC / they have excess capacity（生产）。 “Which is true for BOTH perfect competition and monopolistic competition in the long run?” → zero economic profit（P = ATC） 和 MR = MC；P = MC、在 min ATC 生产只属于完全竞争。

WORKED EXAMPLE 5美甲店：从垄断竞争到“只发 5 张执照” 原创

A city has 60 nail salons. Each offers its own designs, service, and location, and opening a new salon is easy. (a) Identify the market structure. (b) A celebrity makes nail art popular, and salons earn positive economic profit. In the long run, what happens to the number of salons, the demand facing a typical salon, and its economic profit? (c) Compare a typical salon's long-run price and output with those of a perfectly competitive firm with the same costs. (d) Suppose instead the city requires a license and issues only 5 licenses. Can salons earn positive economic profit in the long run? Explain.

(a) 企业多、产品有差异、进入容易 → 垄断竞争。

(b) 利润吸引新店进入：店数增加，每家店的需求减少（左移，并变得更有弹性），经济利润降到 0。

(c) 价格更高（P = ATC，但不在 ATC 最低点），产量更少（产能过剩）。

(d) 可以。执照是法律上的进入壁垒，新店进不来，利润不会被竞争掉——市场变成了少数几家的寡头（见 4.5），更像上册的垄断：长期经济利润可以大于 0。

练习 Practice 4.4（三）· 长期均衡与效率

以下为历年 AP 真题（带图、表和收益矩阵的题按原题重绘），点选项即时判分并显示解析。

4.4-242000 AP Micro Q42

Monopolistically competitive firms are inefficient because they

produce a lower level of output at a higher average cost than do perfectly competitive firms
use production processes that are more capital-intensive than do perfectly competitive firms
face downward-sloping demand curves, ensuring that marginal revenue is greater than average revenue
produce at that level of output where price equals marginal cost
realize diseconomies of scale

长期在 ATC 最低点左边生产（产能过剩）：与完全竞争企业相比，产量更少、平均成本更高。(C) MR 小于 AR，不是大于；(D) 是完全竞争。

4.4-252005 AP Micro Q11

Which of the following is true of a monopolistically competitive firm in long-run equilibrium?

Price is greater than marginal cost, and marginal revenue is equal to average total cost.
Price is greater than marginal revenue, and marginal cost is equal to average total cost.
Price is greater than marginal revenue, and marginal cost is greater than average total cost.
Marginal revenue is equal to marginal cost, and price is equal to average total cost.
Marginal revenue is greater than marginal cost, and price is equal to average total cost.

长期：利润最大 MR = MC，经济利润为 0 P = ATC（相切）。(B)(C) 切点在 ATC 下降段，此时 MC < ATC，不是 MC = ATC。

4.4-262012 AP Micro Q54

Let P = price, MR = marginal revenue, MC = marginal cost, and ATC = average total cost. In monopolistic competition, which of the following most accurately describes the long-run equilibrium conditions for a firm?

P > ATC, MR = MC, and P > MC
P > ATC, MR > MC, and P = MC
P = ATC, MR = MC, and P > MC
P = ATC, MR = MC, and P = MC
P = ATC, MR > MC, and P > MC

长期均衡：P = ATC（零利润），MR = MC（利润最大），P > MC（需求向下倾斜） → (C)。

4.4-272013 AP Micro Q26

Which of the following is true for a monopolistically competitive firm in long-run equilibrium?

It earns a normal profit.
It sets price equal to marginal cost.
It is allocatively efficient.
It is a price taker.
It produces at minimum average total cost.

进出自由 → 长期经济利润为 0 → 只赚正常利润。(B)(C) P > MC，没有配置效率；(E) 不在 min ATC 生产。

4.4-282014 AP Micro 练习卷 Q30

Monopolistically competitive firms are considered inefficient in allocating society’s resources for which of the following reasons?

In equilibrium, the marginal benefit exceeds the price charged by the firms.
In equilibrium, the marginal benefit exceeds the marginal cost of production.
In equilibrium, the marginal revenue of the firm is not equal to its marginal cost.
In long-run equilibrium, the firm is earning economic profits.
Firms exhibit significant market power and therefore the number of firms in the industry is strictly limited.

均衡时价格 = 消费者的边际收益 MB，而 P > MC → MB > MC：再多生产一单位，社会得到的比花掉的多，产量太少。

4.4-292014 AP Micro 练习卷 Q56

Which of the following is true of a monopolistically competitive firm in long-run equilibrium?

Price equals marginal cost and average total cost.
Price equals average total cost but is greater than marginal cost.
Price equals marginal cost and is greater than average total cost.
The firm makes no attempt to differentiate its products.
The firm earns positive economic profits by producing at minimum average cost.

长期：P = ATC（零利润），但 P > MC（需求向下倾斜，P > MR = MC）。

4.4-302015 AP Micro 练习卷 Q25

Which of the following is true of a monopolistically competitive firm in long-run equilibrium?

Price exceeds marginal revenue, and the firm earns positive economic profits.
Price equals marginal revenue, and the firm earns positive economic profits.
Price equals marginal cost, and the firm earns zero economic profits.
Price exceeds marginal cost, and the firm earns zero economic profits.
Price exceeds marginal cost, and the firm earns positive economic profits.

长期 P > MC，经济利润 为 0。(A)(B)(E) 长期没有正利润；(C) P = MC 是完全竞争。

4.4-312016 AP Micro 练习卷 Q37

Which of the following is true of a monopolistically competitive firm in long-run equilibrium?

The firm produces the allocatively efficient level of output.
The firm is allocatively inefficient, because it produces an output level at which price is greater than marginal cost.
The firm produces an output level that minimizes average total cost.
The firm produces in the inelastic range of its demand curve.
The firm earns positive economic profits but zero accounting profits.

长期 P > MC → 配置无效率。(C) 不在 min ATC；(D) 企业在需求的弹性区生产（MR = MC > 0）；(E) 正好反了。

4.4-322018 AP Micro 练习卷 Q24

Monopolistically competitive product markets are inefficient because

price equals the marginal value to the buyer of the last item produced
price is greater than marginal cost
excessive competition prevents other firms from entering the market
homogeneous goods are usually overpriced
short-run economic profit-making opportunities exist

没有效率是因为 P > MC。(A) 说的正是有效率的条件；(D) 垄断竞争的产品有差异，不是同质。

4.4-332018 AP Micro 练习卷 Q55

Which of the following is true of a monopolistically competitive firm in long-run equilibrium?

It produces where price equals marginal cost, and it earns zero economic profits.
It produces where marginal revenue exceeds marginal cost, and it earns positive economic profits.
It produces where marginal cost equals marginal revenue, and the price is equal to average total cost.
It produces at the minimum average total cost, and it utilizes all excess capacity.
It produces more than the allocatively efficient quantity, and the price is greater than marginal cost.

长期：MC = MR 且 P = ATC。(A) P = MC 是完全竞争；(D) 有产能过剩；(E) 产量少于配置有效产量。

4.4-342019 AP Micro 练习卷 Q55

Firms in monopolistic competition do not attain allocative efficiency because at the long-run equilibrium output, which of the following is true?

Price is greater than marginal cost.
Marginal cost is greater than minimum average total cost.
Marginal revenue is greater than marginal cost.
There is an overallocation of resources to the market.
Products are homogeneous.

配置效率要求 P = MC；长期均衡时 P > MC。(D) 资源配置不足（underallocation），不是过多。

4.4-352010 AP Micro Q40

Which of the following statements is true for both a monopolistically competitive firm and a perfectly competitive firm in long-run profit-maximizing equilibrium?

Economic profits equal zero, and price equals marginal cost.
Economic profits equal zero, and price equals marginal revenue.
Marginal revenue equals marginal cost, and profits are positive.
Economic profits equal zero, and marginal revenue equals marginal cost.
Economic profits equal zero, and price exceeds marginal cost.

两种结构的长期均衡都满足 零经济利润（进出自由）和 MR = MC（利润最大化）。P = MC、P = MR 只对完全竞争成立。

4.4-362015 AP Micro 练习卷 Q10

Which of the following is true for both a monopolistically competitive firm and a perfectly competitive firm in long-run equilibrium?

Marginal cost is greater than marginal revenue.
Price is greater than marginal cost.
Price is equal to average total cost.
Price is equal to marginal cost.
Marginal revenue is equal to average revenue.

两者长期都是零经济利润 → P = ATC。(B) 只有垄断竞争；(D)(E) 只有完全竞争。

4.5Oligopoly and Game Theory 寡头与博弈论

全世界的大型客机几乎只有波音和空客两家在造；手机操作系统基本只剩 iOS 和 Android。在这样的市场里，每家企业做决定之前都要先想：“我这样做，对手会怎么回应？”——这就是寡头，而分析“猜对手”的工具叫博弈论。

Characteristics 寡头的特征

定义与特征（CED PRD-3.C.1） Oligopoly：少数几家企业占据市场（两家时叫 duopoly 双头垄断），而且：

进入壁垒高：规模经济、巨额启动成本、专利、牌照、网络效应等，新企业很难进来，所以经济利润在长期也可能保持。
相互依存（interdependent）：一家的利润直接取决于对手的选择，所以每家都要考虑对手的反应——这是寡头区别于其他三种结构的最主要特征。
产品可以相同（钢铁、水泥、原油）也可以有差异（汽车、手机、碳酸饮料）。
没有效率：价格高于边际成本（P > MC），产量低于社会最优。

✈️

大型客机：波音与空客。造一款新机型要投入上百亿美元，规模经济巨大。

📱

手机操作系统：用户越多，开发者越愿意为它写 App，App 越多又吸引更多用户（网络效应）。

🥤

可乐：可口可乐与百事。一家推出新口味或降价促销，另一家往往很快跟进。

📶

电信运营商：牌照和基站网络是很高的进入壁垒，市场上只有少数几家。

某国碳酸饮料市场（虚构数据）：最大的四家企业占了 83% 的销售额（four-firm concentration ratio = 83%），剩下几十家小厂分 17%。少数几家占大头 → 寡头。AP 选择题会用“前三/前四家企业占 85%、88% 的市场”来描述寡头。

易错 TRAP · 寡头与垄断竞争怎么分看企业数量和进入壁垒，不看产品有没有差异（两种结构的产品都可以有差异）：很多家、进出自由 → 垄断竞争；少数几家、壁垒高、互相盯着 → 寡头。题目说“firms consider each other's decisions / recognize their mutual interdependence / game theory is useful” → 一定是寡头。

Collusion and Cartels 合谋与卡特尔（CED PRD-3.C.2）

定义 DEFINITION Collusion 合谋：几家企业不再互相竞争，而是协调价格或产量，像一家垄断企业那样行事，以获得最大的总利润。
Cartel 卡特尔：以正式协议来合谋的企业集团。最有名的例子是石油输出国组织 OPEC，成员国约定各自的产油量。
在美国等许多国家，企业之间的合谋定价是违法的（反垄断法，见 Unit 6）。有时企业不明说，却心照不宣地维持高价，叫默契合谋（tacit collusion），常见形式是价格领导（price leadership）：一家先定价，其他家跟随。合谋破裂、大家竞相降价，就是价格战（price war）。

合谋能让大家都多赚钱，为什么还经常破裂？看一个例子：

WORKED EXAMPLE 6海岛上的两家制冰厂：守约还是偷偷多卖？原创

Polar Ice and Frosty Ice are the only two ice makers on a hot island. Market demand is P = 20 − Q, where Q is hundreds of bags per day. Each firm's marginal cost and average total cost are constant at $4 per bag.

(a) If the two firms form a cartel and act like a single monopoly, find the market quantity, the price, and each firm's profit when they split output equally. (b) Frosty keeps the agreement and produces 4 (hundred bags), but Polar secretly produces 5. Find the new price and each firm's profit. (c) Each firm can produce 4 or 5. Complete the payoff matrix, identify each firm's dominant strategy, and identify the Nash equilibrium. (d) Compare the Nash equilibrium price and quantity with the cartel outcome and with perfect competition.

(a) 卡特尔像垄断：MR = 20 − 2Q = MC = 4 → Q = 8，P = 20 − 8 = $12。总利润 = (12 − 4) × 8 = 64（百美元），各产 4、各得 32（即 $3,200/天）。

(b) Q = 4 + 5 = 9，P = $11。Polar：(11 − 4) × 5 = 35（比守约多 3）；Frosty：(11 − 4) × 4 = 28（少了 4）；总利润 63 < 64。多卖的那一份，好处全归作弊者，降价的损失两家分摊——这就是作弊的动机。

(c) 都产 5：Q = 10，P = $10，各得 (10 − 4) × 5 = 30。

		Frosty
		Produce 4	Produce 5
Polar	Produce 4	$32, $32	$28, $35
Polar	Produce 5	$35, $28	$30, $30

单位：百美元/天。第一个数是 Polar 的利润，第二个数是 Frosty 的利润。

Polar：Frosty 产 4 时 35 > 32，Frosty 产 5 时 30 > 28 → 不管对方怎么做，产 5 都更好 → 优势策略：产 5。Frosty 同理。纳什均衡 = (产 5, 产 5)，各得 30。

(d) 纳什均衡 Q = 10、P = $10：比卡特尔（Q = 8、P = $12）产量多、价格低，但比完全竞争（P = MC = $4、Q = 16）产量少、价格高。两家都守约本来各得 32，却都落到 30——典型的囚徒困境。

两家合起来像一个垄断企业：MR = MC 时 Q = 8、P = $12，总利润 64（百美元），各分 32。如果两家都偷偷多产 1（Q = 10），价格跌到 $10；完全竞争则会做到 P = MC = $4、Q = 16。

为什么卡特尔不稳定在卡特尔价格下 P > MC：多卖一单位，作弊者自己拿到 P − MC 的利润，而价格下降的损失由所有成员分摊。所以每个成员都有“偷偷多卖一点”的动机；如果大家都这样做，价格下跌，人人的利润都比守约时少。

哪些情况让合谋更难维持

企业数量多：每家作弊对价格的影响小，又更难互相监督。
进入容易：高价会吸引新企业进来，新企业不受协议约束。
难以监督：价格和销量不公开、产品和定价方式复杂，作弊不容易被发现。
成本或利益不同：各家想要的价格不一样，很难谈拢；买方实力强、需求波动大，也会让协议破裂。

LAB 2守约还是偷偷多卖？ · Cartel Cheating

海岛上只有 Polar 和 Frosty 两家制冰厂。市场需求 P = 20 − Q（Q：百袋/天），每袋成本都是 $4。两家约定各产 4。拖动两家的产量，看价格、各自利润和总利润怎么变。

Polar 产量 = 4

Frosty 产量 = 4

练习 Practice 4.5（一）· 寡头、合谋与卡特尔

以下为历年 AP 真题（带图、表和收益矩阵的题按原题重绘），点选项即时判分并显示解析。

4.5-12012 AP Micro Q39

Which of the following best describes an oligopolistic market?

Many sellers with identical products and no barriers to entry
Many sellers, each with a clearly differentiated product, and no barriers to entry
A few competing sellers with similar products and high barriers to entry
A few competing sellers of identical products and no barriers to entry
No competition among sellers and high barriers to entry

寡头：少数几家相互竞争的卖家、产品相似（或相同）、进入壁垒高。(A) 完全竞争；(B) 垄断竞争；(E) 垄断。

4.5-22023 AP Micro 国际卷 Q36

Which of the following best represents the most distinctive characteristic of an oligopoly?

A market with a single firm that faces no competition
A market where a few firms have significant market share
A market with low barriers to entry
A market with many firms selling differentiated products
A market with many firms earning zero economic profits in the long run

寡头最突出的特征：少数几家企业占有很大的市场份额。(A) 垄断；(D)(E) 垄断竞争。

4.5-32024 AP Micro Q15

Which of the following best describes an oligopoly?

CoffeeBar can sell as many cups of coffee as it wants at the market price but will lose all its customers if it charges a higher price.
CoffeeBar is the only coffee shop in town with complete market power to charge high prices and earns positive economic profit in the long run.
CoffeeBar is the only employer of beverage service workers in town and pays a fixed wage to all hired workers.
CoffeeBar sets its own price for its coffee beverages, but because it is one of many coffee shops in town, it earns zero economic profit in the long run.
CoffeeBar is one of only two coffee shops in town and is constantly offering promotions and discounts to steal customers from the other coffee shop.

镇上只有两家咖啡店、不停促销抢对方的顾客 → 少数企业、相互依存 → 寡头。(A) 完全竞争；(B) 垄断；(C) 买方垄断；(D) 垄断竞争。

4.5-42010 AP Micro Q13

If the three largest widget producers control 85 percent of the total widget market, then these producers are operating in

an oligopoly
monopolistic competition
perfect competition
a monopoly
a cartel

最大的三家占 85% → 少数企业主导 → 寡头。(E) 卡特尔需要正式的合谋协议，题目没说。

4.5-52019 AP Micro 练习卷 Q12

If the four largest firms in a market produce 88 percent of total industry output, the market is

perfectly competitive
a pure monopoly
a natural monopoly
an oligopoly
a monopsony

前四家占 88% 的产量 → 少数企业主导 → 寡头。

4.5-62000 AP Micro Q41

Interdependence among firms is a characteristic primarily associated with

labor markets
perfect competition
monopsony
oligopoly
monopoly

相互依存（一家的决策影响其他家）是寡头的标志。垄断只有一家，完全竞争企业太多，彼此都不必考虑。

4.5-72012 AP Micro Q11

In which of the following market structures do firms recognize their mutual interdependence?

Oligopoly
Monopoly
Perfect competition
Unregulated natural monopoly
Monopsony

少数几家企业才会意识到彼此的相互依存 → 寡头。

4.5-82014 AP Micro 练习卷 Q10

The characteristic of oligopolistic firms that makes them different from all other types of firms is that oligopolistic firms

are regulated by a state agency or federal agency
consider each other’s decisions
advertise their products
produce differentiated products
produce identical products

寡头区别于其他所有企业的特征：考虑彼此的决策。(C)(D) 垄断竞争也做广告、也有差异化产品。

4.5-92015 AP Micro 练习卷 Q55

Collusion, price leadership, and price wars are usually observed in which of the following market structures?

Perfect competition
Monopolistic competition
Oligopoly
Monopoly
Natural monopoly

合谋、价格领导、价格战都是少数企业相互依存时的行为 → 寡头。

4.5-102022 AP Micro 国际卷 Q2

Which of the following best describes firms in an industry if all the firms are in a cartel?

They produce the allocatively efficient quantity.
They face a perfectly elastic demand curve.
They have identical cost curves.
They coordinate their production decisions.
They agree to remove barriers to entry.

卡特尔 = 正式合谋的企业集团 → 它们协调生产决策（限产以抬价）。(A) 合谋会减产，没有配置效率；(E) 卡特尔希望壁垒越高越好。

4.5-112005 AP Micro Q23

If the only two firms in an industry successfully collude to maximize their joint profit, the price for the product will be

equal to the marginal cost of production
equal to the average total cost of production
above the marginal cost of production
above the monopoly price
below the average variable cost of production

成功合谋 = 像垄断一样在 MR = MC 处定产量 → 价格高于边际成本。(D) 合谋最多达到垄断价格，不会更高。

4.5-122000 AP Micro Q52

Which of the following best explains why it is difficult to maintain lasting collusive agreements?

There is an unavoidable conflict in that a collusive agreement can increase the profits of some, but not all, firms in the industry.
There is little potential for gain from collusion unless there is a large number of consumers in the market.
Each firm in the industry views itself as facing a vertical demand curve, even though the market demand curve is downward sloping.
The firms in the industry have a common incentive to increase output to a more competitive level.
Each firm realizes that its profits would increase if it were the only firm to violate the collusive agreement by increasing its production slightly.

卡特尔不稳定的原因：每家都知道，如果只有自己偷偷多产一点，自己的利润会增加。人人这样想，协议就破裂。

4.5-132013 AP Micro Q41

A cartel is difficult to maintain for which of the following reasons?

Consumers substitute away from the good when the price increases.
Individual cartel members are tempted to cheat on the agreement.
Although the total gain to cartel members is positive, all members lose when everyone sticks to the agreement.
Some firms will reduce output in an effort to lower costs of production.
Oligopolistic behavior is generally predictable.

成员有作弊的诱惑：卡特尔价格高于 MC，偷偷多卖的每一单位都赚钱。(C) 守约时所有成员都比竞争时赚得多。

4.5-142010 AP Micro Q56

A collusive agreement to fix prices among firms in an oligopolistic industry is most likely to be broken under which of the following conditions?

It is easy for new firms to enter into the industry.
All of the firms have identical costs.
The number of firms is few.
Firms' sales are widely reported.
The market demand is stable.

进入容易：高价会吸引新企业，新企业不受协议约束，会抢走市场 → 合谋最容易破裂。(B)–(E) 成本相同、企业少、销量公开、需求稳定，都让合谋更容易维持。

Games and Payoff Matrices 博弈与收益矩阵（CED PRD-3.C.3–C.4）

博弈的四个要素

Game 博弈：几个参与者各自行动，每个人的收益同时取决于自己和别人的选择。
Players 参与者：AP 只考两个参与者。
Strategy 策略：一个完整的行动计划（定高价/定低价、做广告/不做广告……）。AP 只考每人两个策略。
Payoff 收益：每种策略组合下每个人得到的结果（通常是利润）。把所有组合的收益列成表，就是收益矩阵（payoff matrix，也叫 normal form）。

读矩阵的约定：行参与者在左边，列参与者在上面；每格第一个数是行参与者的收益，第二个数是列参与者的收益。本讲义用蓝色表示行参与者、橙色表示列参与者。

		Frosty
		Produce 4	Produce 5
Polar	Produce 4	$32, $32	$28, $35
Polar	Produce 5	$35, $28	$30, $30

例：右上角 ($28, $35) = Polar 产 4、Frosty 产 5 时，Polar 赚 $28，Frosty 赚 $35。

易错 TRAP · 比较时只看自己的数判断行参与者怎么选：固定一列，上下比较第一个数。判断列参与者怎么选：固定一行，左右比较第二个数。最常见的错误是拿行参与者的收益去左右比较。

Dominant Strategy 优势策略（CED PRD-3.C.5）

定义 DEFINITION Dominant strategy：不管对方选什么，这个策略给自己的收益都更高。有优势策略的参与者根本不用猜对手——选它就对了。
Best response 最优反应：给定对方某个选择时，自己收益最高的策略。如果两种情况下的最优反应是同一个策略，它就是优势策略；如果最优反应随对方而变，就没有优势策略。

WORKED EXAMPLE 7元旦健身房要不要打折？原创

FitBox and IronHouse are the only two gyms in a small town. Each gym decides whether to offer a New Year discount. The matrix shows their profits in thousands of dollars; the first entry is FitBox's profit and the second is IronHouse's. They choose simultaneously and independently, and both know all the payoffs.

		IronHouse
		Discount	No Discount
FitBox	Discount	$30, $20	$50, $25
FitBox	No Discount	$25, $35	$45, $32

(a) If IronHouse offers a discount, what is FitBox's best response? (b) Does FitBox have a dominant strategy? Explain using numbers from the matrix. (c) Does IronHouse have a dominant strategy? Explain. (d) Identify the Nash equilibrium. (e) If the two gyms merged and kept both locations, which combination would maximize their combined profit?

		IronHouse
		Discount	No Discount
FitBox	Discount	$30, $20	$50, $25
FitBox	No Discount	$25, $35	$45, $32

(a) 看 IronHouse = Discount 那一列的第一个数：Discount 30 > No Discount 25 → Discount。

(b) 有，Discount：IronHouse 打折时 30 > 25，不打折时 50 > 45——不管对方怎么做，打折都更好。

(c) 没有：FitBox 打折时，IronHouse 不打折更好（25 > 20）；FitBox 不打折时，IronHouse 打折更好（35 > 32）——最优选择随对方而变。

(d) FitBox 一定打折；给定这一点，IronHouse 选 No Discount（25 > 20）→ 纳什均衡 (Discount, No Discount)，利润 $50k 与 $25k。

(e) 四格合计：50、75、60、77 → 合并后选 (No Discount, No Discount)，合计 $77k。合并后的企业只看总利润，不再关心“谁多谁少”，所以结果可以不是原来的纳什均衡。

Nash Equilibrium 纳什均衡（CED PRD-3.C.6）

定义 DEFINITION Nash equilibrium：在这个策略组合下，给定对方的选择，没有任何一方能通过单独改变自己的策略来提高收益。换句话说，双方的选择互为最优反应，谁也不后悔。

最快的找法是划线法：把每个参与者在每种情况下的最优反应划线，两个数都被划线的格子就是纳什均衡。点按钮一步一步看：

		Frosty
		Produce 4	Produce 5
Polar	Produce 4	$32, $32	$28, $35
Polar	Produce 5	$35, $28	$30, $30

① 找 Polar 的最优反应：先固定 Frosty 的选择，在每一列里比较 Polar（第一个数）的收益：Frosty 产 4 时，35 > 32；Frosty 产 5 时，30 > 28。把较大的那个划线——两列都是“产 5”，所以产 5 是 Polar 的优势策略。

		Frosty
		Produce 4	Produce 5
Polar	Produce 4	$32, $32	$28, $35
Polar	Produce 5	$35, $28	$30, $30

② 找 Frosty 的最优反应：再固定 Polar 的选择，在每一行里比较 Frosty（第二个数）的收益：Polar 产 4 时，35 > 32；Polar 产 5 时，30 > 28。划线——Frosty 的优势策略也是产 5。

		Frosty
		Produce 4	Produce 5
Polar	Produce 4	$32, $32	$28, $35
Polar	Produce 5	$35, $28	$30, $30

③ 两个数都被划线的格子：两个数都被划线的格子，就是双方互为最优反应——纳什均衡：(产 5, 产 5)，各得 $30。

箭头表示“换一个策略能赚更多”。每个格子只要有箭头从它出发，就有人想离开，就不是均衡。只有右下角（都产 5）没有箭头出去——这是唯一的纳什均衡，尽管左上角（都产 4）对两家都更好。

优势策略与纳什均衡的关系

双方都有优势策略 → 双方都选它，这个格子一定是（唯一的）纳什均衡（“dominant strategy equilibrium”）。
只有一方有优势策略 → 它选优势策略，另一方对它做最优反应。
都没有优势策略 → 用划线法逐格检查：可能有 2 个均衡，也可能 1 个都没有。
纳什均衡不一定是双方总收益最大的格子，也不一定对双方最好。

WORKED EXAMPLE 8点球与约会：没有均衡，或者有两个原创

(a) In a penalty kick, the kicker chooses Left or Right and the goalie dives Left or Right at the same moment. The kicker wins (payoff 1) if the two choose different sides; the goalie wins (payoff 1) if the two choose the same side; the loser gets 0. Is there a Nash equilibrium? (b) Ann and Ben want to meet on Saturday but their phones are dead. Each chooses the Library or the Cafe. If they go to the same place, Ann gets 10 at the Library and 8 at the Cafe, while Ben gets 8 at the Library and 10 at the Cafe; if they miss each other, both get 0. Find all Nash equilibria. Does either player have a dominant strategy?

(a) 逐格检查：(左, 左) 门将扑中，射手想改右；(左, 右) 进球，门将想改左……每一格都有人想换 → 没有纳什均衡（AP 范围内只考“纯策略”均衡，所以答“没有”即可）。

点球（1 = 赢，0 = 输）：箭头绕成一圈，每个格子都有人想换 → 没有纳什均衡。

(b) (Library, Library) 和 (Cafe, Cafe) 都是纳什均衡：在那里谁单独换地方都会从 8 或 10 掉到 0。两人都没有优势策略——最好的去处取决于对方去哪儿。

约在哪儿见面（数字是开心程度）：两个绿格都没有人想离开 → 两个纳什均衡。

规律：2 × 2 博弈可能有 0 个、1 个或 2 个纳什均衡。只有当双方都有优势策略时，才能直接确定唯一的均衡。

AP 考试范围（CED 排除项）只考两个参与者、每人两个策略、同时行动的博弈。多于两人或两个策略的博弈、混合策略、博弈树（先后行动）都不考。

LAB 3点格子找纳什均衡 · Find the Nash Equilibria

选一个博弈，点你认为是纳什均衡的格子（可以点多个，也可以一个都不点），再点“检查”。卡住了就先点“划线”看双方的最优反应。（已答对 0 / 个博弈）

练习 Practice 4.5（二）· 博弈论与优势策略

以下为历年 AP 真题（带图、表和收益矩阵的题按原题重绘），点选项即时判分并显示解析。

4.5-152013 AP Micro Q9

The use of game theory to explain strategic behavior among firms is most associated with which of the following market structures?

Perfect competition
Monopolistic competition
Oligopoly
Monopoly
Monopsony

博弈论分析的是相互依存的策略行为 → 寡头。

4.5-162010 AP Micro Q26

Game theory is a useful model to explain the behavior of firms in a market when the firms are

independent of one another
price takers
regulated by government
altruistic
interdependent

每家的收益取决于对手的选择时才需要博弈论 → 企业相互依存。

4.5-172018 AP Micro 练习卷 Q10

Game theory is most useful in describing outcomes in markets where

firms are price takers
there is only one producer
there are many small producers
products are identical for all firms
there are interdependent firms

博弈论最适合相互依存的企业。(A)(C)(D) 完全竞争的企业彼此影响不到；(B) 只有一家就没有对手。

4.5-182017 AP Micro 练习卷 Q7

Game theory is used to explain

why firms price discriminate
how monopolies evolve into oligopolies
strategic behavior of firms in oligopoly
profit maximization in monopoly
price leadership of monopolistic competition

博弈论用来解释寡头企业的策略行为。(E) 价格领导是寡头现象，不是垄断竞争。

4.5-192023 AP Micro 国际卷 Q12

A player’s dominant strategy is

independent of the other player’s choice
dependent on the other player’s choice
the strategy that results in the maximum combined payoff for both players
the strategy that always delivers the other player’s best outcome of the game
the strategy that always delivers the other player’s worst outcome of the game

优势策略：不管对方怎么选都是最好的 → 与对方的选择无关（independent）。

4.5-202025 AP Micro 国际卷 Q54

Ally’s Candles and Bob’s Candles are the only two sellers of candles in town. Both candle companies are considering changing their current pricing strategies. Each company is deciding between increasing prices or decreasing prices to increase their profits. Suppose Ally’s Candles realizes that increasing price will increase its profits regardless of what Bob’s Candles does. Which of the following concepts best describes this situation?

Nash equilibrium
Price discrimination
Dominant strategy
Prisoner’s Dilemma
Collusion

“不管 Bob 怎么做，涨价都让 Ally 的利润增加” → 这正是优势策略的定义。

4.5-212018 AP Micro 练习卷 Q57

In a market with two firms, a firm that has a dominant strategy will do which of the following?

Maintain that strategy independent of the strategies chosen by its competitor.
Adjust its strategies based on the strategies chosen by its competitor.
Make the first move, and wait to see whether its competitor responds to the move.
Keep its competitor guessing about its next move.
Collude with its competitor to reduce uncertainty.

有优势策略的企业，不管对手怎么选都坚持这个策略，不需要随对手调整。

4.5-222010 AP Micro Q39

		Tara
		High	Low
Pam	High	$40, $40	$20, $70
Pam	Low	$70, $20	$30, $30

按原题重绘 · 第一个数是 Pam 的收益

Pam and Tara run two competing lemonade stands in a town. In the payoff matrix above, the first entry in each cell shows the profits to Pam, and the second entry in each cell shows the profits to Tara. According to the information, which of the following is true?

If Pam sets the high price, Tara will do best by charging the low price.
If Pam sets the low price, Tara will do best by charging the high price.
The dominant strategy for both is to charge the high price.
The dominant strategy for Tara is to set the low price and for Pam is to set the high price.
Neither Tara nor Pam has a dominant strategy.

Pam 定高价时，Tara：High $40 vs Low $70 → Low，(A) 对。(B) Pam 定低价时 Tara：20 vs 30 → 还是 Low；两人的优势策略都是 Low（70 > 40，30 > 20），所以 (C)(D)(E) 错。

4.5-232013 AP Micro Q37

Question 37 is based on the information below.

The table below shows the profits associated with the strategies of two oligopolistic firms, Lock and Star, that must choose between a high price and a low price for their products. The first entry in each box is the profits received by Lock, and the second entry is the profits received by Star.

		Star
		High Price	Low Price
Lock	High Price	$70, $70	−$20, $50
Lock	Low Price	$50, −$20	$40, $40

按原题重绘 · 第一个数是 Lock 的收益

If Lock chooses to charge the low price, the best course of action for Star would be to charge the

low price and earn a profit of $40
low price and earn a profit of $50
high price and earn a profit of $50
high price and earn a profit of $70
high price and incur a loss of $20

Lock 定低价（看下面一行）时，Star：High −$20 vs Low $40 → 定低价，赚 $40。（这个博弈双方都没有优势策略：对方高我就高、对方低我就低，有两个纳什均衡。）

4.5-242012 AP Micro Q40

The payoff matrix below shows the per-unit profits associated with the production strategies of two utility companies, UA and UB. Each firm has two choices: to reduce production by 10 percent or by 20 percent. The first entry in each cell indicates the profits to UA, and the second, the profits to UB.

		UB
		Reduces Production by 20 Percent	Reduces Production by 10 Percent
UA	Reduces Production by 20 Percent	$150, $150	$50, $250
UA	Reduces Production by 10 Percent	$250, $50	$100, $100

按原题重绘 · 第一个数是 UA 的收益

Based on the information, and assuming no cooperation, which of the following statements is true?

Neither company has a dominant strategy.
Both companies have an incentive to reduce production by 10%.
Both companies have an incentive to reduce production by 20%.
Only UA has an incentive to reduce production by 20%.
Only UB has an incentive to reduce production by 20%.

UA：UB 减 20% 时，减 10% 赚 250 > 150；UB 减 10% 时，减 10% 赚 100 > 50 → 减 10% 是优势策略；UB 对称 → 两家都减产 10%。（都减 20% 各得 150 更好——囚徒困境。）

4.5-252014 AP Micro 练习卷 Q40

The following table shows the profits associated with the pricing strategies of two oligopolistic firms, Agronomia and Farmingdale. Each firm has two possible strategies: to charge a low price or a high price. The first entry in each cell shows the profits to Agronomia and the second the profits to Farmingdale.

		Farmingdale
		Farmingdale charges high price.	Farmingdale charges low price.
Agronomia	Agronomia charges high price.	$150, $150	$50, $300
Agronomia	Agronomia charges low price.	$300, $50	$100, $100

按原题重绘 · 第一个数是 Agronomia 的收益

If the two firms do not cooperate, as a result of the firms’ pricing decisions the profits of each firm will be which of the following?

Agronomia’s Profit: $50；Farmingdale’s Profit: $100
Agronomia’s Profit: $150；Farmingdale’s Profit: $150
Agronomia’s Profit: $300；Farmingdale’s Profit: $50
Agronomia’s Profit: $100；Farmingdale’s Profit: $100
Agronomia’s Profit: $300；Farmingdale’s Profit: $300

Agronomia：对方高价时低价 300 > 150，对方低价时低价 100 > 50 → 低价是优势策略；Farmingdale 对称 → 都定低价，各得 $100, $100（都定高价本可各得 150）。

4.5-262015 AP Micro 练习卷 Q37

Assume that Alpha and Beta are the only sellers of a product and they do not cooperate. Each firm has to decide whether to raise the product price. The payoff matrix below gives the profits, in dollars, associated with each pair of pricing strategies. The first entry in each cell shows the profits to Alpha, and the second, the profits to Beta.

		Beta
		Raise Price	Do Not Raise Price
Alpha	Raise Price	$100, $100	$30, $120
Alpha	Do Not Raise Price	$110, $20	$50, $70

按原题重绘 · 第一个数是 Alpha 的收益

Assuming both firms know the information in the matrix, which of the following correctly describes the dominant strategy of each firm?

Alpha: Do not raise price；Beta: Do not raise price
Alpha: Do not raise price；Beta: Raise price
Alpha: Raise price；Beta: No dominant strategy
Alpha: Raise price；Beta: Do not raise price
Alpha: No dominant strategy；Beta: Raise price

Alpha：Beta 涨价时不涨 110 > 100；Beta 不涨时不涨 50 > 30 → 不涨价。Beta：Alpha 涨价时不涨 120 > 100；Alpha 不涨时不涨 70 > 20 → 不涨价。

4.5-272016 AP Micro 练习卷 Q39

E Soda and R Soda are the only two firms in the soft-drink industry. The companies cannot cooperate. Each firm can follow a high-price strategy or a low-price strategy for pricing its product. In the payoff matrix below, the first entry in each cell shows the profits to E Soda and the second entry shows the profits to R Soda.

		R Soda
		Low Price	High Price
E Soda	Low Price	$25, $25	$90, $15
E Soda	High Price	$15, $90	$75, $75

按原题重绘 · 第一个数是 E Soda 的收益

Given the information in the payoff matrix, it can be concluded that

neither E Soda nor R Soda has a dominant strategy
E Soda has a dominant strategy but R Soda does not
R Soda has a dominant strategy but E Soda does not
both firms will choose the high-price strategy
both firms will choose the low-price strategy

E Soda：R 低价时低价 25 > 15，R 高价时低价 90 > 75 → 低价是优势策略；R Soda 对称 → 两家都定低价（都定高价各得 75 本来更好）。

Prisoner's Dilemma 囚徒困境（CED PRD-3.C.7）

囚徒困境 PRISONER'S DILEMMA 每个参与者都有优势策略，可是双方都选优势策略的结果（纳什均衡），却比双方都“合作”的结果对两人都差。个人的理性选择导致集体的坏结果。

身边的囚徒困境

🧊

卡特尔作弊：制冰厂都守约各赚 32，可偷偷多产总是更划算，最后都只赚 30。

📺

广告战：两家都打广告，顾客在两家之间的分配和都不打时差不多，却白白多花了广告费。1971 年美国禁止香烟的电视广告后，有研究认为这反而帮烟草公司省下了一场广告战的开支。

🎤

演唱会站起来：前排一站，后排只好也站，最后大家都站着，谁也没看得更清楚，还更累。

🚀

军备竞赛：两国都扩军，相对实力不变，却都在军费上花了大钱。

这正是寡头难以维持垄断结果的原因（CED PRD-3.C.7）。不过寡头的结果仍然比完全竞争“差”：价格更高、产量更低。

同一个市场的三种结果：寡头各自追求利润，达不到垄断（卡特尔）的结果；但价格仍高于完全竞争、产量仍低于完全竞争（CED PRD-3.C.7）。

制冰市场	Q	P	两家总利润	消费者剩余	DWL
卡特尔（= 垄断）	8	$12	64	32	32
寡头纳什均衡（各产 5）	10	$10	60	50	18
完全竞争	16	$4	0	128	0

单位：百美元/天。消费者剩余 = ½ × (20 − P) × Q；DWL = ½ × (P − 4) × (16 − Q)。

延伸：重复博弈（了解即可）现实中的企业不是只“博弈一次”，而是天天打交道。如果对手采取以牙还牙（tit-for-tat）——先合作，之后对方上一次怎么做我就怎么做——那么今天作弊多赚的一点，会换来以后长期的价格战。所以重复互动可以支撑默契合谋，这也是许多寡头市场价格能长期高于竞争水平的原因之一。

Changing Payoffs and Mergers 收益改变、激励与合并

近年 FRQ 的博弈题，后两问几乎总是：“某个收益变了（征税、补贴、罚款、成本变化、一方给另一方钱），重画矩阵 / 新的纳什均衡是什么”“收益至少要变多少，某个策略才会成为优势策略”“两家合并后最大总利润是多少”。

三类问法的做法

重画矩阵：只改受影响的那一方、那几格的数字；没受影响的数原样照抄，玩家名字和策略名也要写上。
至少要多少激励（CED PRD-3.C 学习目标 c）：对每一种“对方的选择”算出需要的差额，取较大的那个，而且要严格大于它（等于时两个策略一样好，不是严格的优势策略）。
合并 / 合谋：合并后的企业只关心总利润 → 把每格两个数相加，选最大的一格。

WORKED EXAMPLE 9电车还是油车：补贴要多大才能改变选择？原创

Aurora Motors and Bison Auto are the only two carmakers in a country. Each decides whether its new model will be Electric or Gas. The matrix shows profits in millions of dollars (Aurora's first). They choose simultaneously and independently with complete information.

		Bison
		Electric	Gas
Aurora	Electric	$40, $40	$50, $60
Aurora	Gas	$70, $45	$55, $50

(a) Identify each firm's dominant strategy, if any, and the Nash equilibrium. (b) The government will give a subsidy to any firm that chooses Electric. What is the minimum subsidy that would make Electric a dominant strategy for Aurora? (c) Suppose the subsidy is $10 million for each firm that chooses Electric. Redraw the matrix and identify all Nash equilibria. (d) Without any subsidy, if the two firms merged, which combination would maximize their combined profit?

(a) Aurora：Bison 选 Electric 时 Gas 70 > 40，Bison 选 Gas 时 Gas 55 > 50 → 优势策略 Gas。Bison：Aurora 选 Electric 时 Gas 60 > 40，Aurora 选 Gas 时 Gas 50 > 45 → 优势策略 Gas。纳什均衡 (Gas, Gas) = ($55, $50)。

(b) 补贴 s 加在 Aurora 选 Electric 的两格上。要让 Electric 在两列都更好：40 + s > 70 且 50 + s > 55 → s > 30 且 s > 5 → 补贴必须超过 $30 million。注意要满足“对方任何选择下都更好”，所以取两个门槛中较大的那个。

(c) 只有选 Electric 的那一方的收益加 10：

		Bison
		Electric	Gas
Aurora	Electric	$50, $50	$60, $60
Aurora	Gas	$70, $55	$55, $50

补贴 $10 million 后。左上：双方都 +10；右上：只有 Aurora +10；左下：只有 Bison +10；右下不变。

Aurora：Bison 选 Electric 时 Gas 70 > 50；Bison 选 Gas 时 Electric 60 > 55 → 没有优势策略了。Bison 同理。两个纳什均衡：(Electric, Gas) = ($60, $60) 和 (Gas, Electric) = ($70, $55)——$10 million 不足以让 Electric 成为优势策略，但足以让“一家做电车”成为均衡。

(d) 四格合计：80、110、115、105 → 合并后选 Aurora Gas、Bison Electric，合计 $115 million。

LAB 4罚多少才不打补贴战？ · Changing the Payoffs

两家网约车平台 Swift 和 Zoom 决定要不要给乘客发补贴（单位：百万美元，第一个数是 Swift 的利润）。监管部门对每一家发补贴的平台罚款 $f。拖动罚款，看矩阵、优势策略和纳什均衡怎么变；再看看如果两家合并，会选哪一格。

罚款 f = $0 million

FRQ 答题模板 · Explain using numbers from the matrix

最优反应：“If Firm B chooses X, Firm A earns $a by choosing Y, which is greater than $b by choosing Z, so A's best response is Y.”
优势策略：两句都写——“If B chooses X, A earns more with Y ($a > $b); if B chooses Z, A also earns more with Y ($c > $d).”
没有优势策略：写出两种情况下最优反应不同，并各给一组数字。
是不是纳什均衡：两方都要检查——“Given B's choice, A cannot do better…; given A's choice, B cannot do better…”。

练习 Practice 4.5（三）· 纳什均衡

以下为历年 AP 真题（带图、表和收益矩阵的题按原题重绘），点选项即时判分并显示解析。

4.5-282019 AP Micro 练习卷 Q22

		Town Herald
		Do Not Change Subscription Price	Increase Subscription Price
Daily Voice	Do Not Change Subscription Price	$500, $400	$200, $700
Daily Voice	Increase Subscription Price	$600, $300	$300, $100

按原题重绘 · 第一个数是 Daily Voice 的收益

The two major newspapers in a city, Daily Voice and Town Herald, are considering whether to raise the subscription price. The first entries in the matrix above show the profits to Daily Voice, and the second entries show the profits to Town Herald. Which of the following is consistent with the above payoff matrix?

Do Not Change Subscription Price is a dominant strategy for Daily Voice.
Do Not Change Subscription Price is a dominant strategy for Town Herald.
Increase Subscription Price is a dominant strategy for Daily Voice.
Increase Subscription Price is a dominant strategy for Town Herald.
There are no dominant strategies in the above payoff matrix.

Daily Voice：Town Herald 不变价时，涨价 600 > 500；Town Herald 涨价时，涨价 300 > 200 → 涨价是 Daily Voice 的优势策略。Town Herald：DV 不变时涨价更好（700 > 400），DV 涨价时不变更好（300 > 100）→ 没有优势策略。

4.5-292019 AP Micro 练习卷 Q37

When two firms interact in an oligopolistic market, which of the following statements is true?

If one firm has a dominant strategy, then the other firm does not have a dominant strategy.
If one firm has a dominant strategy, then the other firm also has a dominant strategy.
Both firms must have dominant strategies.
If one firm has a dominant strategy, then there is no Nash equilibrium.
If both firms have dominant strategies, then there is a Nash equilibrium.

如果双方都有优势策略，双方都选它，这个组合下谁也不想单独改变 → 一定存在纳什均衡。(A)(B)(C) 一方有没有优势策略与另一方无关；(D) 一方有优势策略时仍可有纳什均衡。

4.5-302022 AP Micro 国际卷 Q10

The project funding decisions made by two interdependent research companies, Creative and Genezyz, resulted in a profit of $4 million for Creative and a profit of $5 million for Genezyz. Which of the following is required for this combination of project choices to be a Nash equilibrium?

Genezyz charges a lower price to its customers than Creative does.
Neither firm has a dominant strategy.
Neither firm could earn higher profits by unilaterally changing its project choice.
Creative could earn a higher profit if it chose a different project when Genezyz chose its initial project.
The combined profits of the two firms are maximized at the current combination of project choices.

纳什均衡的定义：任何一方都不能通过单独改变选择提高利润。(E) 纳什均衡不一定使总利润最大；(B) 有没有优势策略都可能有纳什均衡。

4.5-312022 AP Micro 国际卷 Q24

RKB and JCM are two competing firms that are considering which project to develop. RKB will develop either Project Alpha or Project Beta. JCM will develop either Project X or Project Y. The payoff matrix provided shows the payoffs for each combination of strategies, and both players (RKB and JCM) have complete information. The first value in each cell represents the payoff for RKB, and the second value represents the payoff for JCM. Each firm independently and simultaneously chooses its strategy.

		JCM
		Project X	Project Y
RKB	Project Alpha	$300, $400	$250, $250
RKB	Project Beta	$350, $250	$400, $300

按原题重绘 · 第一个数是 RKB 的收益

Which of the following conclusions is supported by the information in the payoff matrix?

There is no combination of project choices that results in a Nash equilibrium.
There are multiple combinations of project choices that result in Nash equilibria.
RKB has a dominant strategy, but JCM does not.
JCM has a dominant strategy, but RKB does not.
Neither firm has a dominant strategy.

RKB：JCM 选 X 时 Beta 350 > 300，选 Y 时 Beta 400 > 250 → RKB 的优势策略是 Beta。JCM：RKB 选 Alpha 时 X 更好（400 > 250），选 Beta 时 Y 更好（300 > 250）→ 没有。纳什均衡只有一个 (Beta, Y)。

4.5-322017 AP Micro 练习卷 Q19

Questions 19-20 refer to the table below.

The two firms in an industry are deciding whether to advertise. The profit to each firm depends on the other firm’s decision. The first entries in the matrix below indicate the profit earned, in millions of dollars, by Firm A; and the second entries indicate the profits earned, in millions of dollars, by Firm B.

		Firm B
		Advertise	Do Not Advertise
Firm A	Advertise	$7, $1	$5, $4
Firm A	Do Not Advertise	$3, $2	$2, $0

按原题重绘 · 第一个数是 Firm A 的收益

Based on the payoff matrix, which of the following is correct?

Firm A always gets a smaller share of the industry profits.
Firm A’s dominant strategy is to advertise.
Firm B’s dominant strategy is not to advertise.
The dominant strategy for both firms is not to advertise.
Neither firm has a dominant strategy.

Firm A：B 做广告时 7 > 3，B 不做时 5 > 2 → A 的优势策略是做广告。Firm B：A 做广告时不做更好（4 > 1），A 不做时做更好（2 > 0）→ 没有优势策略。(A) A 的份额并不总是更小。

4.5-332017 AP Micro 练习卷 Q20

		Firm B
		Advertise	Do Not Advertise
Firm A	Advertise	$7, $1	$5, $4
Firm A	Do Not Advertise	$3, $2	$2, $0

按原题重绘 · 第一个数是 Firm A 的收益

The combination where Firm A advertises and Firm B does not advertise is Nash equilibrium because

it is best for each firm given what the other firm has chosen
the total industry profits are maximized
Firm A has an incentive to change its strategy and chooses not to advertise
it is the best outcome for Firm B regardless of what firm A does
advertising is always the best strategy for Firm A

(A 做广告, B 不做) = ($5, $4)：给定 B 不做，A 做广告 5 > 2；给定 A 做广告，B 不做 4 > 1 → 双方都是对对方的最优反应。这个格子总利润恰好也最大（9），但那不是它成为纳什均衡的原因。

4.5-342016 AP Micro 练习卷 Q54

		Nature View
		Bids High	Bids Low
Evergreen	Bids High	$600, $300	$520, $400
Evergreen	Bids Low	$720, $100	$500, $200

按原题重绘 · 第一个数是 Evergreen 的收益

Evergreen and Nature View are bidding for a landscaping contract. The payoff matrix above shows what each firm’s total weekly profits from all its operations will be for each combination of bids. The first entry in each cell shows Evergreen’s profit, and the second entry in each cell shows Nature View’s profit. A Nash equilibrium results under which of the following conditions?

When Evergreen bids low, no matter what Nature View’s bid is
When both firms bid high and when both firms bid low
When Evergreen bids high and Nature View bids low
When both firms bid low
When both firms bid high

Nature View：E 高价时低价 400 > 300，E 低价时低价 200 > 100 → 低价是优势策略。给定 NV 低价，Evergreen：高价 520 > 低价 500 → Evergreen 高价、Nature View 低价。

4.5-352018 AP Micro 练习卷 Q25

North Springs and South Springs are two firms that serve the local market for bottled water. Each firm must choose to either maintain or increase its current output. The first entry in each cell in the payoff matrix below shows the profits for North Springs, and the second entry in each cell shows the profits for South Springs.

		South Springs
		Maintain Output	Increase Output
North Springs	Maintain Output	$15, $30	$15, $45
North Springs	Increase Output	$30, $26	$27, $28

按原题重绘 · 第一个数是 North Springs 的收益

If the two firms do not cooperate, which of the following represents the payoff North Springs and South Springs receive in the dominant-strategy equilibrium and the Nash equilibrium?

Dominant Strategy: None；Nash Equilibrium: ($27, $28)
Dominant Strategy: ($27, $28)；Nash Equilibrium: ($15, $45)
Dominant Strategy: ($30, $26)；Nash Equilibrium: ($15, $45)
Dominant Strategy: ($27, $28)；Nash Equilibrium: ($27, $28)
Dominant Strategy: ($30, $26)；Nash Equilibrium: ($27, $28)

North Springs：对方维持时增产 30 > 15，对方增产时增产 27 > 15 → 增产是优势策略；South Springs：45 > 30、28 > 26 → 增产是优势策略。优势策略均衡 = 纳什均衡 = (增产, 增产) = ($27, $28)。

4.5-362025 AP Micro 国际卷 Q17

There are two barbecue restaurants in a town: Nick’s Barbecue and Frank’s Barbecue. The payoff matrix shows the payoffs for each combination of strategies, and both players have complete information. The first entry in each cell represents Nick’s Barbecue’s payoff, and the second entry represents Frank’s Barbecue’s payoff. Each player independently and simultaneously chooses its strategy.

		Frank’s Barbecue
		Advertise	Not Advertise
Nick’s Barbecue	Advertise	$600, $100	$360, $250
Nick’s Barbecue	Not Advertise	$370, $140	$350, $150

按原题重绘 · 第一个数是 Nick’s Barbecue 的收益

Which of the following is true about the restaurants’ dominant strategies?

Nick’s Barbecue’s dominant strategy is Advertise, and Frank’s Barbecue’s dominant strategy is Advertise.
Nick’s Barbecue’s dominant strategy is to Advertise, and Frank’s Barbecue’s dominant strategy is to Not Advertise.
Nick’s Barbecue’s dominant strategy is Not Advertise, and Frank’s Barbecue’s dominant strategy is Advertise.
Nick’s Barbecue’s dominant strategy is to Not Advertise, and Frank’s Barbecue’s dominant strategy is to Not Advertise.
Nick’s Barbecue’s dominant strategy is Advertise, and Frank’s Barbecue does not have a dominant strategy.

Nick：Frank 做广告时做广告 600 > 370，Frank 不做时做广告 360 > 350 → Advertise。Frank：Nick 做广告时不做 250 > 100，Nick 不做时不做 150 > 140 → Not Advertise。

ΣSummary 本册总结与易错点

4.4 垄断竞争 · 特征与短期

企业很多、产品有差异、进出自由。需求向下倾斜但较有弹性，MR < P；MR = MC 定产量，从需求读价格。短期可能盈利、亏损或正常利润；P < AVC 停业。广告让需求增加、更缺乏弹性，广告费提高 ATC。

4.4 垄断竞争 · 长期与效率

盈利 → 进入 → 每家需求左移；亏损 → 退出 → 需求右移；直到需求与 ATC 相切：MR = MC，P = ATC（零利润），P > MC。切点在 ATC 最低点左边 → 产能过剩，既无配置效率也无生产效率；价格高于、产量低于完全竞争。

4.5 寡头 · 合谋与卡特尔

少数企业、进入壁垒高、相互依存，P > MC。合谋（卡特尔）让它们像垄断一样限产抬价；但卡特尔价格下 P > MC，每家都有偷偷多卖的动机，所以卡特尔不稳定。企业多、进入容易、难以监督时更容易破裂。

4.5 博弈论

收益矩阵：第一个数属于行参与者。最优反应：固定对方选择比较自己的收益。优势策略：不管对方选什么都最好。纳什均衡：互为最优反应，没人想单独改变（划线法：两个数都被划线）。可能有 0、1、2 个纳什均衡。囚徒困境：都选优势策略，结果对双方都更差。

4.5 收益改变与合并

重画矩阵只改受影响的数。让某策略成为优势策略的最小激励 = 各种情况下所需差额中较大的那个，且要严格大于。合并后选两数之和最大的格子。寡头的结果介于垄断与完全竞争之间：价格高于、产量低于完全竞争。

Unit 4 Panorama 四种市场结构全景

	完全竞争	垄断竞争	寡头	垄断
企业数量	非常多	很多	少数几家（相互依存）	一家
产品	完全相同	有差异	相同或有差异	没有相近替代品
进入壁垒	没有	很低	高	非常高
企业的需求	水平，P = MR	向下倾斜，较有弹性，MR < P	向下倾斜，取决于对手	市场需求，MR < P
产量规则	MR = MC，价格从需求读（完全竞争 P = MR，所以就是 P = MC）
P 与 MC	P = MC	P > MC	P > MC	P > MC
长期经济利润	0	0（需求与 ATC 相切）	可以 > 0	可以 > 0
配置效率	有	无	无	无（完全价格歧视除外）
生产效率（长期）	有（min ATC）	无（产能过剩）	一般无	一般无
典型例子	小麦、鸡蛋批发	餐馆、理发店、服装品牌	客机、手机系统、可乐	自来水、专利新药
讲义位置	Unit 3（下）	4.4（本册）	4.5（本册）	4.2–4.3（上册）

易错点 TOP 10

垄断竞争企业的需求向下倾斜是因为企业多 ✗ → 是因为产品有差异。
广告让需求更有弹性 ✗ → 广告让需求更缺乏弹性（并增加需求）。
垄断竞争长期均衡 P = MC ✗ → P = ATC > MC，而 MR = MC。
长期相切点就是 ATC 最低点 ✗ → 在最低点左边（MC < ATC），所以有产能过剩。
长期利润为 0，所以垄断竞争有效率 ✗ → P > MC（配置无效率），不在 min ATC（生产无效率）。
垄断竞争企业时刻盯着对手的反应 ✗ → 那是寡头的特征（相互依存）；垄断竞争企业太多，不必考虑某一个对手。
卡特尔对所有成员都有利，所以很稳定 ✗ → 每个成员都有作弊的动机，卡特尔容易破裂。
找行参与者的最优反应时左右比较 ✗ → 固定对方的选择（一列），上下比较第一个数。
纳什均衡就是总收益最大或对双方最好的格子 ✗ → 只是“没有人想单独改变”；囚徒困境里它对双方都更差。
“至少要多少钱才能让某策略成为优势策略”取较小的差额 ✗ → 要在对方的每一种选择下都更好，取较大的差额，并且严格大于。

✎Unit 4（下）Test 本册测验

15 道选择题，全部选自历年 AP 真题（与各节练习不重复，带图和收益矩阵的题按原题重绘），建议用时 18 分钟。先全部选完，再点底部“交卷”查看得分和解析。

T12025 AP Micro 国际卷 Q33

Brass With Class is a profit-maximizing firm that produces trumpets in a monopolistically competitive market. Brass With Class currently produces 100 trumpets and sells each trumpet at a price of $50. If the average total cost when producing 100 trumpets is $45, what is Brass With Class’s total economic profit in the short run, and what adjustment will happen in the trumpet market in the long run?

Economic Profit: −$500；Long-Run Adjustment: Existing firms will exit the market.
Economic Profit: −$5；Long-Run Adjustment: Existing firms will exit the market.
Economic Profit: $0；Long-Run Adjustment: The market will be in long-run equilibrium.
Economic Profit: $5；Long-Run Adjustment: New firms will enter the market.
Economic Profit: $500；Long-Run Adjustment: New firms will enter the market.

利润 = (50 − 45) × 100 = $500 > 0 → 进入壁垒低，长期新企业进入，直到经济利润为 0。

T22017 AP Micro 练习卷 Q48

Questions 47-48 refer to the following graph, which shows the cost and revenue curves for a profit-maximizing monopolistically competitive firm.

按原题重绘

Is the firm in short-run or long-run equilibrium?

Short run, because price is greater than marginal cost
Short run, because the firm is earning a positive economic profit
Long run, because price is greater than average total cost
Long run, because marginal revenue is not equal to zero
Either short run or long run, because the firm is producing where marginal revenue equals marginal cost

在 Q₁ 处价格 P₅ 高于 ATC（ATC 在 P₂ 与 P₃ 之间） → 有正的经济利润，会吸引进入 → 这只能是短期均衡。(A) 垄断竞争长期也有 P > MC，不能用来区分。

T32024 AP Micro Q46

Which of the following is a difference between monopolies and firms in monopolistic competition?

Firms in monopolistic competition can earn negative economic profit in the short run, but monopolies cannot.
Firms in monopolistic competition set marginal revenue equal to marginal cost to maximize profit, but monopolies do not.
Firms in monopolistic competition produce the socially optimal quantity of output in the long run, but monopolies do not.
Monopolies charge a price that is greater than marginal cost, but firms in monopolistic competition do not.
Monopolies can earn positive economic profit in the long run, but firms in monopolistic competition cannot.

垄断有进入壁垒，长期可以保持正的经济利润；垄断竞争进出自由，长期利润为 0。(A) 垄断短期也会亏损；(B) 两者都 MR = MC；(C)(D) 两者都 P > MC、产量低于社会最优。

T42023 AP Micro 国际卷 Q10

Daily Tools is a typical firm in a monopolistically competitive market that produces tool kits for everyday use. Which of the following explains why Daily Tools is inefficient in long-run equilibrium?

The entry of more efficient firms is blocked.
The firm produces the quantity at which marginal cost equals average total cost.
The firm does not produce the quantity that results in minimum average total cost.
The firm does not produce the quantity that minimizes total cost.
The firm’s customers value the last unit of output at less than its cost.

长期均衡时需求与 ATC 在 ATC 下降段相切 → 没有在 ATC 最低的产量生产（产能过剩，生产无效率）。(E) 正好反了：顾客对最后一单位的评价高于成本。

T52021 AP Micro 国际卷 Q34

Which of the following is true about price (P), marginal cost (MC), marginal revenue MR, and economic profit of a monopolistically competitive firm in long-run equilibrium?

P>MC; Marginal revenue is below demand; Economic profit is zero
P<MC; Marginal revenue is below demand; Economic profit is positive
P>MC; Marginal revenue is above demand; Economic profit is positive
P=MR=MC; Marginal revenue is equal to demand; Economic profit is zero
P=MR=MC; Marginal revenue is above demand; Economic profit is positive

长期：P > MC，MR 曲线在需求下方，经济利润 为 0 → (A)。

T62024 AP Micro Q52

The reason profit-maximizing monopolistically competitive firms in long-run equilibrium have excess capacity is

they charge a price equal to marginal cost
they produce a quantity at which average total cost is not minimized
they produce a quantity at which price equals marginal revenue
they produce a quantity at which marginal cost is greater than average total cost
they are experiencing diseconomies of scale

产能过剩 = 产量小于使 ATC 最低的产量，原因是它在 ATC 没有达到最低的产量上生产。(D) 切点在 ATC 下降段，此时 MC < ATC。

T72016 AP Micro 练习卷 Q13

Game theory is most commonly used for analyzing the pricing behavior of firms in which market structure?

Perfect competition
Monopolistic competition
Oligopoly
Monopoly
Monopsony

博弈论最常用来分析寡头的定价行为。

T82017 AP Micro 练习卷 Q38

Which of the following characteristics is prevalent in oligopolies?

Allocative efficiency
Low barriers of entry
Consideration of rivals’ reactions
No deadweight loss
Zero economic profit

寡头的普遍特征：考虑对手的反应。(A)(D) 寡头 P > MC，有 DWL；(B) 壁垒高；(E) 长期利润可以为正。

T92018 AP Micro 练习卷 Q39

There are four firms in an oligopolistic industry. The four firms agree to collude and act like a monopoly. If one of the firms violates the agreement and charges a lower price or sells a larger quantity than what was agreed to, what will happen in the short run?

The firm that cheats will earn higher profits, and industry profits will be lower.
The firm that cheats will earn higher profits, and industry profits will be higher.
The firm that cheats will earn lower profits, and industry profits will be lower.
The firm that cheats will earn lower profits, and industry profits will be higher.
The firms that do not cheat will earn higher profits.

作弊者多卖、价格下降：作弊者自己的利润上升，但整个行业的产量偏离了垄断产量，行业总利润下降（守约的企业受损）。

T102025 AP Micro 国际卷 Q39

If the only two firms in an industry collude to charge a higher price and produce a lower quantity than what is socially optimal, what will happen to consumer surplus and total economic surplus in the market in the short run?

Consumer surplus will decrease, and total economic surplus will stay the same.
Consumer surplus will decrease, and total economic surplus will increase.
Consumer surplus will increase, and total economic surplus will decrease.
Consumer surplus and total economic surplus will both increase.
Consumer surplus and total economic surplus will both decrease.

合谋提高价格、减少产量 → 消费者剩余减少；产量低于社会最优，出现无谓损失 → 总剩余也减少。

T112021 AP Micro 国际卷 Q40

Evergreen and Nature View are bidding for a landscaping contract. The payoff matrix provided shows what each firm’s total weekly profit will be for each combination of bids. The first entry in each cell shows the profits of Evergreen, and the second entry shows the profits of Nature View. Evergreen and Nature View act independently and simultaneously, and each firm knows all the information about the payoffs associated with the strategies. Which of the following describes their dominant strategies?

		Nature View
		Bids High	Bids Low
Evergreen	Bids High	$5, $5	$0, $6
Evergreen	Bids Low	$6, $0	$3, $3

按原题重绘 · 第一个数是 Evergreen 的收益

Evergreen does not have a dominant strategy. Nature View’s dominant strategy is to bid high.
Evergreen’s dominant strategy is to bid high. Nature View does not have a dominant strategy.
Evergreen’s dominant strategy is to bid low. Nature View’s dominant strategy is to bid high.
Evergreen’s dominant strategy is to bid high. Nature View’s dominant strategy is to bid high.
Evergreen’s dominant strategy is to bid low. Nature View’s dominant strategy is to bid low

Evergreen：对方高价时低价 6 > 5，对方低价时低价 3 > 0 → 低价是优势策略；Nature View 对称 → 两家的优势策略都是 bid low（典型囚徒困境：都出高价各得 5 更好）。

T122024 AP Micro Q57

Bygone2 and Cleared4U are two competing firms in the waste management industry. When deciding to invest in future technologies, each firm can choose to open a new recycling center or a new composting center. The payoff matrix provided shows the value of profits for each combination of strategies, and both firms have complete information. The first value in each cell represents the payoff for Bygone2, and the second value represents the payoff for Cleared4U. Each firm independently and simultaneously chooses its strategy. Which of the following statements is true?

		Cleared4U
		Recycling	Composting
Bygone2	Recycling	$45, $47	$42, $37
Bygone2	Composting	$38, $43	$40, $41

按原题重绘 · 第一个数是 Bygone2 的收益

Neither firm has a dominant strategy.
Both firms have a dominant strategy to open a new composting center.
Both firms have a dominant strategy to open a new recycling center.
Cleared4U has a dominant strategy to open a new recycling center, and Bygone2 has a dominant strategy to open a new composting center.
Bygone2 has a dominant strategy to open a new recycling center, and Cleared4U has a dominant strategy to open a new composting center.

Bygone2：对方 Recycling 时 45 > 38，对方 Composting 时 42 > 40 → Recycling；Cleared4U：对方 Recycling 时 47 > 37，对方 Composting 时 43 > 41 → Recycling → 两家的优势策略都是建回收中心。

T132023 AP Micro 国际卷 Q46

Questions 46-47

Rover’s Rest and Dave’s Den are the only two dog daycare companies in a town. Each firm is considering whether to run ads or to not run ads. The first entry in each cell shows Dave’s Den’s profit, and the second entry shows Rover’s Rest’s profit. Each dog daycare company independently and simultaneously chooses its strategy. Assume the two firms know all the information in the matrix and do not cooperate. Use the payoff matrix provided to answer the next two questions.

		Rover’s Rest
		Run Ads	No Ads
Dave’s Den	Run Ads	$500, $400	$400, $500
Dave’s Den	No Ads	$700, $800	$900, $700

按原题重绘 · 第一个数是 Dave’s Den 的收益

Which of the following represents the Nash equilibrium?

Dave’s Den chooses “Run Ads,” and Rover’s Rest chooses “No Ads.”
Both Dave’s Den and Rover’s Rest choose “Run Ads.”
Both Dave’s Den and Rover’s Rest choose “No Ads.”
Dave’s Den chooses “No Ads,” and Rover’s Rest chooses “Run Ads.”
There are multiple Nash equilibria.

Dave：Rover 做广告时不做 700 > 500，Rover 不做时不做 900 > 400 → Dave 的优势策略是 No Ads。给定 Dave 不做，Rover：做广告 800 > 不做 700 → Dave No Ads，Rover Run Ads（$700, $800）。

T142023 AP Micro 国际卷 Q47

		Rover’s Rest
		Run Ads	No Ads
Dave’s Den	Run Ads	$500, $400	$400, $500
Dave’s Den	No Ads	$700, $800	$900, $700

按原题重绘 · 第一个数是 Dave’s Den 的收益

The advertising agency raises its prices, causing a firm that chooses to run ads to earn $200 less in profits. Once the firms adjust to the higher advertising prices, what is the total combined profit of the two firms in the new Nash equilibrium?

$500
$700
$1,300
$1,500
$1,600

每个选 Run Ads 的一方利润少 $200，重画矩阵：

		Rover’s Rest
		Run Ads	No Ads
Dave’s Den	Run Ads	$300, $200	$200, $500
Dave’s Den	No Ads	$700, $600	$900, $700

广告涨价后（每个选 Run Ads 的一方少 $200）

Dave：No Ads 仍是优势策略（700 > 300，900 > 200）。给定 Dave 不做广告，Rover：不做 700 > 做 600 → 新纳什均衡 (No Ads, No Ads)，总利润 900 + 700 = $1,600。

T152021 AP Micro 国际卷 Q20

Downtown Tech Company and Uptown Tech Company compete in the gaming console market. Downtown Tech Company is deciding whether to charge a high price or a low price for its product. Uptown Tech Company is deciding whether to invest in research and development or advertising. Each company’s profit depends on the actions taken by the other firm, which are listed in the payoff matrix provided. The first entry in the matrix is Downtown Tech Company’s profit, and the second is Uptown Tech Company’s profit. Each firm independently and simultaneously selects an action. Each firm knows all of the information about the payoffs associated with the strategies each firm can choose.

		Uptown Tech Company
		Research and Development	Advertising
Downtown Tech Company	High Price	$500, $250	$100, $700
Downtown Tech Company	Low Price	$200, $400	$600, $350

按原题重绘 · 第一个数是 Downtown Tech Company 的收益

The Nash equilibrium to this game, if one exists, is which of the following?

High Price, Research and Development
Low Price, Advertising
High Price, Advertising
Low Price, Research and Development
None of the strategy pairs represents a Nash equilibrium.

划线：Downtown 在 Uptown 选 R&D 时选 High（500 > 200），选 Advertising 时选 Low（600 > 100）；Uptown 在 Downtown 选 High 时选 Advertising（700 > 250），选 Low 时选 R&D（400 > 350）。没有哪一格两个数都被划线 → 没有纳什均衡。

已作答 0 / 15

FRQFree-Response Practice 自由问答

前三道是 AP 真题，评分标准依照 College Board 官方 Scoring Guidelines 整理；第四道是依照 AP 题型原创的垄断竞争画图题。先在纸上完整作答（包括画图和重画矩阵），再展开对照。

FRQ 1 · 2021 AP Micro FRQ Set 2 Q3 · 5 分 · AZY Foods 的广告真题

The table below shows the total cost and total benefit of advertisements placed by AZY Foods, a firm in the retail food market.

Number of Advertisements	Total Cost ($)	Total Benefit ($)
1	300	1,200
2	500	2,200
3	800	3,000
4	1,300	3,600
5	2,100	4,000
6	3,000	4,200
7	4,100	4,200

(a) Calculate the total net benefit of placing three advertisements. Show your work.
(b) Calculate the marginal net benefit of the third advertisement. Show your work.
(c) What is the optimal number of advertisements placed by AZY Foods? Explain using marginal analysis.
(d) Suppose over the next year the marginal benefit that AZY Foods receives from each advertisement increases by $300. Identify the optimal number of advertisements.
(e) There are many firms in the retail food market. Each firm places its own firm-specific advertisements without considering the actions of its competitors. In what market structure is AZY Foods operating?

第几条广告	1	2	3	4	5	6	7
MB	1,200	1,000	800	600	400	200	0
MC	300	200	300	500	800	900	1,100
MB − MC	900	800	500	100	-400	-700	-1,100
MB + 300 − MC	1,200	1,100	800	400	-100	-400	-800

Point	得分要求与解析
(a) 1 pt	总净收益 = $3,000 − $800 = $2,200（要写出算式）。
(b) 1 pt	第 3 条的边际净收益 = ($3,000 − $2,200) − ($800 − $500) = $800 − $300 = $500。
(c) 1 pt	4 条：第 4 条的边际净收益为正（$600 − $500 = $100），第 5 条为负（$400 − $800 = −$400）。
(d) 1 pt	仍是 4 条：第 4 条 $900 > $500，第 5 条 $700 < $800。
(e) 1 pt	垄断竞争（monopolistic competition）：企业很多，各自做广告而不考虑对手的反应（若考虑对手的反应就是寡头）。

对照 4.4 的火锅店例题：那道题 MB 增加 $200 后最优数量从 3 变成 4；这道题 MB 增加 $300 后最优数量不变——一定要逐条重新比较，不能凭感觉。

FRQ 2 · 2019 AP Micro FRQ Set 2 Q3 · 6 分 · Jackpot Florist 与 Boulevard Gardens 真题

		Boulevard
		Delivery	No Delivery
Jackpot	6:00 P.M.	$35, $30	$55, $20
Jackpot	9:00 P.M.	$25, $50	$45, $40

Jackpot Florist and Boulevard Gardens share the market for floral bouquets in Tuliptown. Jackpot is deciding whether to close at 6:00 P.M. or to close at 9:00 P.M., and Boulevard is deciding whether to offer Delivery or No Delivery. The payoff matrix above shows the annual profits for each combination of actions that the firms can choose. The first entry shows Jackpot's profit, and the second entry shows Boulevard's profit. Assume that both firms know all the information in the matrix and act simultaneously and independently.

(a) Is Jackpot's dominant strategy to close at 6:00 P.M., to close at 9:00 P.M., or does it not have a dominant strategy?
(b) Suppose Jackpot chooses to close at 6:00 P.M. and Boulevard chooses No Delivery. Is this the profit-maximizing action by Boulevard? Explain using values from the payoff matrix.
(c) How much profit will Boulevard earn in the Nash equilibrium?
(d) Suppose these two firms merge to form one company with two locations and still face the same actions and payoffs. Which strategies would the new company choose to maximize its combined profit?
(e) Now assume that instead of merging, Jackpot proposes a plan to cooperate with Boulevard. Jackpot would agree to close at 9:00 P.M., and Boulevard would agree to No Delivery. If either firm breaks the agreement, it MUST subtract $12 from its profit that is then added to the other firm's profit.
(i) Assuming the agreement was in effect and enforced, redraw the matrix, including the players, actions, and payoffs.
(ii) Assuming Boulevard is a profit-maximizing firm and there are no antitrust concerns, would Boulevard agree to Jackpot's proposal? Explain using specific values from the redrawn payoff matrix.

Point	得分要求与解析
(a) 1 pt	Jackpot 的优势策略是 6:00 P.M.（Boulevard 选 Delivery 时 35 > 25，选 No Delivery 时 55 > 45）。
(b) 1 pt	不是：Jackpot 6 点关门时，Boulevard 选 Delivery 赚 $30，选 No Delivery 只赚 $20。
(c) 1 pt	$30。Boulevard 的优势策略是 Delivery（30 > 20，50 > 40），纳什均衡是 (6:00 P.M., Delivery) = ($35, $30)。
(d) 1 pt	9:00 P.M. 和 No Delivery：四格合计 65、75、75、85，最大是 $85。
(e)(i) 1 pt	正确重画矩阵（玩家、策略、收益都要写）：违约的一方减 $12，另一方加 $12；双方都违约的格子不变。
(e)(ii) 1 pt	会同意：重画后 Jackpot 的优势策略变成 9 点（37 > 35，45 > 43），Boulevard 的优势策略变成 No Delivery（32 > 30，40 > 38），新均衡下 Boulevard 的利润从 $30 增加到 $40。

		Boulevard
		Delivery	No Delivery
Jackpot	6:00 P.M.	$35, $30	$43, $32
Jackpot	9:00 P.M.	$37, $38	$45, $40

(e)(i) 协议执行后的矩阵：右上格 Jackpot 违约（55 − 12 = 43，20 + 12 = 32）；左下格 Boulevard 违约（25 + 12 = 37，50 − 12 = 38）。

FRQ 3 · 2025 AP Micro FRQ Set 1 Q3 · 5 分 · Tony's Trinkets 与 Bitaly's Bracelets 真题

Tony's Trinkets and Bitaly's Bracelets are the only two firms in a town that produce and sell jewelry. Tony's Trinkets is deciding whether to produce Unique jewelry or Typical jewelry. Bitaly's Bracelets is deciding whether to produce Gold jewelry or Silver jewelry. The payoff matrix shows the payoffs for each combination of strategies. The first entry in each cell shows Tony's Trinkets' profit, and the second entry shows Bitaly's Bracelets' profit. Each firm independently and simultaneously chooses its strategy. Assume that the two firms know all the information in the matrix and do not cooperate.

		Bitaly’s Bracelets
		Gold	Silver
Tony’s Trinkets	Unique	$15, $21	$20, $19
Tony’s Trinkets	Typical	$10, $7	$21, $16

A. Suppose Bitaly's Bracelets chooses to produce Silver jewelry. Is choosing to produce Unique jewelry the best choice for Tony's Trinkets? Explain using numbers from the payoff matrix.
B. Is Bitaly's Bracelets' dominant strategy to produce Gold jewelry, to produce Silver jewelry, or does it not have a dominant strategy? Explain using numbers from the payoff matrix.
C. Identify all Nash equilibria for this game.
D. Suppose Tony's Trinkets' profit from producing Typical jewelry increases regardless of what Bitaly's Bracelets does. What is the minimum amount by which Tony's Trinkets' profit must increase in order for Typical jewelry to become a dominant strategy: $2, $4, $6, $11, or $15?
E. Suppose instead that these two firms now cooperate and merge into one firm to maximize their combined profits. The new firm will have two locations and continue to face the same actions and payoffs. Calculate the new firm's maximum combined profit. Show your work.

Point	得分要求与解析
A 1 pt	No：Bitaly 选 Silver 时，Tony 选 Typical 赚 $21，比选 Unique 的 $20 多。
B 1 pt	没有优势策略：Tony 选 Unique 时 Gold 更好（$21 > $19）；Tony 选 Typical 时 Silver 更好（$16 > $7）。
C 1 pt	两个纳什均衡：(Unique, Gold) 和 (Typical, Silver)。
D 1 pt	$6。Typical 要在两种情况下都更好：对方选 Gold 时需要 10 + x > 15，即 x > 5；对方选 Silver 时 21 + x > 20 已经成立。选项中大于 $5 的最小值是 $6。
E 1 pt	四格合计 36、39、17、37 → 最大合计利润 = $20 + $19 = $39（要写出算式）。

		Bitaly’s Bracelets
		Gold	Silver
Tony’s Trinkets	Unique	$15, $21	$20, $19
Tony’s Trinkets	Typical	$10, $7	$21, $16

划线法：两个绿格就是两个纳什均衡。

FRQ 4 · 原创 · 9 分 · 垄断竞争的拉面店 Noodle Lab 原创

Noodle Lab is a ramen restaurant in a monopolistically competitive market. Noodle Lab is currently earning positive economic profit.

(a) Draw a correctly labeled graph for Noodle Lab showing its demand, marginal revenue, marginal cost, and average total cost curves, and show each of the following.
  (i) The profit-maximizing quantity, labeled Q₁
  (ii) The profit-maximizing price, labeled P₁
  (iii) The area representing economic profit, shaded completely
(b) What will happen to the number of ramen restaurants in the long run? Explain how this affects the demand for Noodle Lab's ramen.
(c) Draw a new correctly labeled graph showing Noodle Lab in long-run equilibrium. Label the profit-maximizing quantity Q_L and the price P_L.
(d) In long-run equilibrium, is Noodle Lab producing the allocatively efficient quantity? Explain.
(e) On your graph in part (c), show the quantity at which average total cost is minimized, labeled Q_A. Is Noodle Lab productively efficient in the long run? Explain.
(f) Noodle Lab starts an advertising campaign that convinces many customers that its ramen is unique. Will the demand for Noodle Lab's ramen become more elastic, become less elastic, or remain unchanged? Explain.

Point	评分要点与解析（原创题，按 AP 评分方式整理）
(a) 1 pt	向下倾斜的需求 D，MR 在 D 下方（同一纵截距）。
(a) 1 pt	画出 MC，在 MR = MC 处标 Q₁。
(a) 1 pt	从 Q₁ 往上到需求标 P₁；ATC 在 Q₁ 处低于 P₁，MC 穿过 ATC 最低点。
(a) 1 pt	完整涂出利润：P₁ 与 Q₁ 处的 ATC 之间、宽为 Q₁ 的矩形。
(b) 1 pt	拉面店数量增加（有经济利润、进入壁垒低）；顾客被新店分走，Noodle Lab 的需求减少（左移）。
(c) 1 pt	需求与 ATC 相切，切点对应 Q_L 和 P_L；MR = MC 也在 Q_L。
(d) 1 pt	No：在 Q_L 处 P_L > MC（价格 = 边际收益 > 边际成本），产量少于配置有效的数量。
(e) 1 pt	Q_A 标在 ATC 最低点（MC 与 ATC 的交点），位于 Q_L 右边；No：Q_L < Q_A，没有在 ATC 最低处生产（产能过剩）。
(f) 1 pt	Less elastic：顾客认为它独一无二，眼中的相近替代品变少，涨价时流失的顾客更少。

(a) 参考图：MR 在需求下方；Q₁ 在 MR = MC 处，P₁ 从需求读；Q₁ 处 ATC 低于 P₁，MC 穿过 ATC 最低点；绿色矩形是经济利润。

(c)(e) 参考图：需求左移到与 ATC 相切，切点对应 Q_L、P_L（MR = MC 也在 Q_L）；Q_A 是 ATC 最低点的产量，在 Q_L 右边，两者之差是产能过剩。

FRQ 答题提醒

垄断竞争图和垄断图一样画：D、MR（在 D 下方）、MC、ATC（MC 穿过 ATC 最低点）；长期图的需求必须与 ATC 相切，切点在 ATC 最低点左边。
收益矩阵题“Explain using numbers”：把比较的两个数都写出来，并说明是在对方的哪个选择下比较的。
重画矩阵：玩家名字、策略名、四格八个数全部写出，没变的数也要抄上。
“minimum amount … to become a dominant strategy”：对方每种选择下各算一次需要的差额，取较大者，并严格大于它。

Unit 4 结束四种产品市场结构到这里就学完了。复习垄断、自然垄断与价格歧视：→ 打开 Unit 4（上）Monopoly and Price Discrimination（与本文件放在同一文件夹）。
下一单元 Unit 5 进入要素市场：企业作为买方雇用劳动力，其中买方垄断（monopsony）是要素市场里的不完全竞争。

JL · AP Microeconomics · Unit 4（下）Monopolistic Competition and Oligopoly · 题目标签：真题历年 AP 考试原题（含 College Board 练习卷、国际卷），答案均对照官方答案键并重新验算（2024 年题目的答案为自行验算） · 原创依照 AP 题型原创