JL · AP MICROECONOMICS

Unit 4（上）· Monopoly and Price Discrimination

垄断与价格歧视 — 不完全竞争入门、垄断、自然垄断与管制、价格歧视 · 对应 CED 4.1–4.3 · Unit 4 选择题占比约 15–22%（上下册合计）
上下册分工 上册（本册）：市场结构的总体对比，以及最极端的一种——垄断（4.1–4.3）。
下册：介于完全竞争和垄断之间的两种结构——垄断竞争与寡头，以及博弈论（4.4–4.5）。下册的企业图都建立在本册的“需求向下倾斜、MR 在需求下方”之上。
从 Unit 3 带过来的工具
  • 任何企业都在 MR = MC 处生产；利润 = (P − ATC) × Q；短期 P < AVC 就停产。
  • 完全竞争的长期均衡 P = MC = min ATC，同时达到配置效率（P = MC）和生产效率（P = min ATC）——这是本单元比较一切的基准。
  • Unit 2 的消费者剩余、生产者剩余和无谓损失：本册要在垄断图上计算它们的面积。
  • 需要复习时：→ 打开 Unit 3（下）Profit and Perfect Competition
本册学习目标 LEARNING OBJECTIVES
  • 比较四种市场结构，说出不完全竞争的特征和进入壁垒 · Describe imperfectly competitive markets and barriers to entry
  • 解释垄断企业为什么 MR < P，从需求画出 MR · Explain and graph marginal revenue for a monopolist
  • 说明 MR 与需求弹性、总收益的关系 · Relate MR to elasticity and total revenue
  • 用 MR = MC 和需求曲线找出垄断的产量、价格和利润 · Find a monopolist's output, price, and profit
  • 在图上计算垄断的 CS、PS、利润和 DWL，并与完全竞争比较 · Calculate surplus and deadweight loss under monopoly
  • 分析定额税和每单位税对垄断企业的影响 · Analyze taxes on a monopolist
  • 画出自然垄断，比较不管制、P = MC、P = ATC 三种定价 · Graph and regulate a natural monopoly
  • 说出价格歧视的条件，分析完全价格歧视的结果 · Explain price discrimination and its effects
Term 术语Meaning 含义中文
Imperfect CompetitionMonopoly, oligopoly, monopolistic competition (and monopsony in factor markets)不完全竞争
Market PowerA firm's ability to affect the price of its product市场势力
Price MakerA firm facing downward-sloping demand that chooses its price价格制定者
Barriers to EntryHigh fixed costs, legal barriers, control of key resources进入壁垒
MonopolyA single seller of a product with no close substitutes垄断
Marginal Revenue (MR)ΔTR/ΔQ; below price for a firm that must cut price to sell more边际收益
Natural MonopolyEconomies of scale over the entire range of market demand自然垄断
Socially Optimal PriceRegulated price P = MC (allocatively efficient)社会最优定价
Fair-Return PriceRegulated price P = ATC (zero economic profit)公平回报定价
Deadweight Loss (DWL)Lost total surplus from producing less than the efficient quantity无谓损失
Price DiscriminationCharging different prices for the same good, not because of cost differences价格歧视
Perfect Price DiscriminationEach buyer pays their maximum willingness to pay完全价格歧视
考试怎么考 选择题：MR 与价格、弹性的关系，从垄断图上读产量、价格、利润、DWL，垄断与完全竞争的比较，自然垄断的两种管制价格，价格歧视的条件和结果。FRQ：垄断图几乎每年出现（画 D、MR、MC、ATC，标 Pm、Qm，涂利润或 DWL），常和自然垄断管制、征税、完全价格歧视一起考。

4.1Introduction to Imperfectly Competitive Markets 不完全竞争市场入门

Unit 3 的完全竞争是一个“理想模型”：企业多到谁也影响不了价格。现实中的大多数企业或多或少都能自己定价——苹果、可口可乐、你家楼下的奶茶店、城市的自来水公司都是这样。这一单元研究的就是这些不完全竞争市场。

企业很多 · 没有定价能力只有一家 · 定价能力最大完全竞争Perfect competition小麦、鸡蛋批发农贸市场的青椒垄断竞争Monopolistic competition奶茶店、理发店洗发水品牌寡头Oligopoly电信运营商大型客机、手机系统垄断Monopoly城市自来水专利期内的新药Unit 3 学了最左边；Unit 4 学右边三种——统称“不完全竞争”（imperfect competition）上册：垄断（4.2）与价格歧视（4.3）　·　下册：垄断竞争（4.4）与寡头（4.5）
市场结构是一条光谱：越往右，企业越少、进入越难、企业自己定价的能力（market power）越大。
定义 DEFINITION（CED PRD-3.B.1） Imperfectly competitive markets 不完全竞争市场：产品市场中的垄断（monopoly）、寡头（oligopoly）、垄断竞争（monopolistic competition），以及要素市场中的买方垄断（monopsony，Unit 5）。
Market power 市场势力：企业影响自己产品价格的能力。完全竞争企业没有市场势力（price taker），不完全竞争企业有（price maker）。

Four Market Structures 四种市场结构对比

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一家
产品完全相同有差异相同或有差异独一无二，没有相近替代品
进入壁垒没有很低高非常高
企业面对的需求水平（D = MR = P）向下倾斜，较平向下倾斜，受对手影响向下倾斜（就是市场需求）
长期经济利润00可能 > 0可能 > 0
在本课程Unit 34.4（下册）4.5（下册）4.2–4.3（本册）

Downward-Sloping Demand 多卖就得降价

完全竞争企业：price taker不完全竞争企业：price makerQuantity (q)Price0d = MR = P$6q₁q₂想卖多少都是 $6Quantity (q)Price0DP₁q₁P₂q₂多卖就得降价
左：完全竞争企业按市场价想卖多少卖多少。右：不完全竞争企业面对向下倾斜的需求——要从 q₁ 多卖到 q₂，就必须把价格从 P₁ 降到 P₂（CED PRD-3.B.2）。这一点是本单元所有图形的出发点。
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小麦农民比市场价贵 1 分钱：买家马上去买别人一模一样的小麦，一斤也卖不出去——需求完全有弹性（水平）。
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你常去的奶茶店每杯涨 1 元：有些顾客转去别家，但喜欢它口味的熟客还会买——销量下降但不会变成 0，需求向下倾斜。
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城市自来水公司涨价：大家只能少用一点水，没有别家可换——它面对的就是整个市场的需求曲线。
P > MC：不完全竞争为什么没效率（CED PRD-3.B.3） 在不完全竞争市场，企业把价格定在高于边际成本的地方。例：一杯奶茶卖 $6，多做一杯的 MC 只要 $2。还有顾客愿意花 $4 买一杯——$4 > $2，卖给他对社会有 $2 的净收益； 但企业要卖给他就得降价，而降价会让所有杯都便宜，企业算下来不划算，于是不卖。这笔本可以实现的收益就丢掉了——这就是无谓损失（deadweight loss），4.2 会在图上算出来。

Barriers to Entry 进入壁垒（CED PRD-3.B.4）

进入壁垒 BARRIERS TO ENTRY 阻止新企业进入一个行业的因素。有了它们，现有企业的经济利润就不会被新企业“抢走”。
  • High fixed / start-up costs：建厂、建网络要花巨额固定成本，加上规模经济，一家大企业成本最低（自然垄断）。例：电网、地铁、自来水。
  • Legal barriers：专利（patent）、版权（copyright）、政府执照或特许经营权（license, franchise）。例：新药、出租车牌照。
  • Exclusive ownership of key resources：控制了生产所必需的资源。例：钻石矿、某种稀有矿产。
WORKED EXAMPLE 1这家企业靠什么挡住对手？ 原创

Identify the barrier to entry in each case.

  1. In some oil-rich countries, only a state-owned company is allowed to drill for oil.
  2. A drug company holds a 20-year patent on a new cancer drug.
  3. One company owns almost all of the world's diamond mines.
  4. Building a city's natural gas pipeline network costs billions, and one network can serve the whole city.
  5. A messaging app is valuable because everyone you know already uses it.
  6. A city issues only 5,000 taxi licenses.
情形进入壁垒CED 的说法
1 国有石油公司政府规定只许一家经营legal barrier
2 新药专利专利（法律保护）legal barrier
3 钻石矿控制关键资源exclusive ownership of key resources
4 天然气管网规模经济 / 高固定成本 → 自然垄断high fixed/start-up costs
5 聊天软件网络效应：用户越多越有价值，新软件很难吸引用户（课外常见说法）
6 出租车牌照政府执照限制数量legal barrier
为什么重要：Unit 3 里，经济利润会吸引新企业进入，把利润压到 0。有了进入壁垒，新企业进不来，垄断企业的经济利润在长期也可以一直存在。
LAB 1判断市场结构 · Which Market Structure?

根据企业数量、产品是否有差异、进入难不难，判断每个市场最接近哪种结构。（得分：0 / ）

练习 Practice 4.1

以下为历年 AP 真题（带图的题按原题重绘），点选项即时判分并显示解析。

4.1-12000 AP Micro Q56
The condition for allocative efficiency is violated when
  • firms are price makers (price searchers)
  • short-run profits exist in a competitive industry
  • price equals average total cost
  • the market demand curve is inelastic in a competitive industry
  • the market demand curve is elastic in a competitive industry
配置效率要求 P = MC。price maker 面对向下倾斜的需求，利润最大时 P > MC → 违反配置效率。(B)(D)(E) 都不影响完全竞争的 P = MC；(C) P = ATC 只说明经济利润为 0。
4.1-22022 AP Micro 国际卷 Q21
Imperfectly competitive markets do not achieve allocative efficiency because profit maximization for each firm occurs when which of the following is true?
  • Price equals average total cost.
  • Price is greater than marginal cost.
  • Price equals marginal revenue.
  • Price is less than marginal benefit.
  • Price is less than marginal revenue.
不完全竞争企业在 MR = MC 处生产，而它的 MR < P，所以 P > MC；配置效率要求 P = MC。
4.1-32024 AP Micro Q27
Akeilah has received a patent for a new device that gives her the exclusive right to produce it. Which characteristic of imperfectly competitive markets is being described?
  • Barriers to entry
  • Economies of scale
  • Mutual interdependence
  • Price discrimination
  • Collusion
专利给予独家生产权，是法律壁垒，属于进入壁垒（barriers to entry）。
4.1-42018 AP Micro 练习卷 Q40
Which of the following can give a firm market power?
  • Having access to common information
  • Producing a standardized or homogeneous product
  • Lacking barriers to entry or exit
  • Having a large number of competitors in the market
  • Having economies of scale in production over the range of market output
在整个市场产量范围内都有规模经济 → 一家企业成本最低（自然垄断），这让它拥有市场势力。其余选项都是完全竞争的特征。
4.1-52014 AP Micro 练习卷 Q39
Which of the following market structures results in allocative efficiency?
  • Monopoly
  • Monopolistic competition
  • Perfect competition
  • Natural monopoly
  • Oligopoly
只有完全竞争在 P = MC 处生产（长期还有 P = min ATC）；其他市场结构都是 P > MC。
4.1-62025 AP Micro 国际卷 Q24
Which of the following differentiates a profit-maximizing firm in a perfectly competitive market from a profit-maximizing firm in an imperfectly competitive market?
  • A firm in a perfectly competitive market: is a price maker；A firm in an imperfectly competitive market: is a price taker
  • A firm in a perfectly competitive market: minimizes average total cost in the short run；A firm in an imperfectly competitive market: maximizes total revenue in the short run
  • A firm in a perfectly competitive market: earns zero economic profit in the short run；A firm in an imperfectly competitive market: does not earn economic profit in the short run
  • A firm in a perfectly competitive market: produces where price equals marginal cost；A firm in an imperfectly competitive market: produces where price is greater than marginal cost
  • A firm in a perfectly competitive market: produces in the elastic region of its demand curve；A firm in an imperfectly competitive market: produces in the inelastic region of its demand curve
完全竞争企业 P = MR = MC；不完全竞争企业 MR < P，所以在 P > MC 处生产。(A) 说反了；(E) 不完全竞争企业在需求的弹性区生产。
4.1-72025 AP Micro 国际卷 Q56
Suppose that Lindser Landscaping is the sole provider of landscaping services in a small city. A new firm, Horticultural Experts, enters the market. Which of the following is most likely to occur?
  • Lindser Landscaping’s demand will become more inelastic.
  • Lindser Landscaping’s total revenues will decrease.
  • Lindser Landscaping’s economic profit will increase.
  • The deadweight loss in the landscaping services market will increase.
  • The market price of landscaping services will increase.
新企业进入，抢走一部分顾客，Lindser 的需求减少（有了替代品，也变得更有弹性）→ 总收益下降。利润会减少，DWL 减少，市场价格下降。
4.1-82025 AP Micro 国际卷 Q58
MarketPrice ($)Quantity Exchanged (units)Deadweight Loss ($)
12510050
230750300
345500100
4503000
56020200
The table shows the market price, quantity of a good exchanged, and the resulting deadweight loss in five different market structures.

Which market displays outcomes that are consistent with those that result under perfect competition?
  • Market 1
  • Market 2
  • Market 3
  • Market 4
  • Market 5
完全竞争的结果没有无谓损失（DWL = 0）→ Market 4。

4.2Monopoly 垄断

定义与特征（CED PRD-3.B.5） Monopoly：整个行业只有一家企业，产品没有相近替代品，存在很高的进入壁垒——垄断之所以存在，正是因为有进入壁垒。
垄断企业面对的需求曲线就是整个市场的需求曲线（向下倾斜），所以它是 price maker：可以选价格，但选了价格，能卖多少由需求决定。

Marginal Revenue 为什么 MR < P

完全竞争企业多卖一个，收入就多 P（MR = P）。垄断企业要多卖一个，必须把所有单位的价格都降下来，所以多卖一个带来的收入比价格少。

QPTR = P × QMR = ΔTR
1$10$10$10
2$9$18$8
3$8$24$6
4$7$28$4
5$6$30$2
6$5$30$0
7$4$28−$2
8$3$24−$4
Ice cream cones (Q)Price ($)0+$7−$3D$83$74数量效应第 4 个卖 $7价格效应：前 3 个每个少收 $1
卖第 4 个冰淇淋：价格要从 $8 降到 $7。绿色 = 新卖出的这一个带来 +$7；红色 = 前 3 个也只能卖 $7，每个少收 $1，共 −$3。所以第 4 个的 MR = 7 − 3 = $4 < P = $7。
MR 的两个来源 多卖一单位：数量效应（新卖的这一单位带来 +P）+ 价格效应（之前每一单位都少收一点）。价格效应是负的，所以对垄断企业 MR < P（第一单位除外）。
Price ($)DMR$12弹性区：MR > 0缺乏弹性区：MR < 0单位弹性Quantity (Q)TR ($)0TR$726TR 最大（MR = 0）
需求 P = 24 − 2Q，MR = 24 − 4Q：MR 与 D 的纵截距相同、斜率是 D 的两倍，正好在需求横截距的一半（Q = 6）处穿过横轴。Q < 6 时需求有弹性，MR > 0，多卖 TR 上升；Q = 6 时单位弹性，TR 最大；Q > 6 时缺乏弹性，MR < 0。
WORKED EXAMPLE 2从需求公式写出 MR 原创

A monopolist faces the demand curve P = 100 − 2Q, and its marginal cost is constant at $20. (a) Write the equation of the MR curve. (b) At what quantity is total revenue maximized, and what is the maximum TR? (c) Find the profit-maximizing quantity and price. (d) At that quantity, is demand elastic or inelastic?

(a) 同一纵截距、斜率加倍：MR = 100 − 4Q。
(b) TR 最大在 MR = 0：100 − 4Q = 0 → Q = 25，价格 P = 100 − 50 = $50，TR = 50 × 25 = $1,250。
(c) MR = MC：100 − 4Q = 20 → Q = 20；价格读需求：P = 100 − 40 = $60（不是 $20）。
(d) Q = 20 < 25，MR = $20 > 0 → 需求富有弹性。利润最大的产量总是在 TR 最大的产量左边。
画 MR 的规则 需求是直线 P = a − bQ 时，MR = a − 2bQ：同一个纵截距，斜率加倍，横截距减半。FRQ 画垄断图时 MR 一定要画在需求下方、从同一点出发。
易错 TRAP · 垄断只在需求的弹性区生产 MR < 0 的地方（缺乏弹性区）多卖反而让 TR 下降，而 MC > 0，所以 MR = MC 一定落在 MR > 0、需求富有弹性的一段。题目问“垄断企业在需求曲线的哪一段生产”，答案是 elastic portion。
LAB 2多卖一个，收入怎么变？ · Marginal Revenue

海岛上唯一的冰淇淋车，需求 P = 11 − Q。拖动滑块决定卖几个：绿色是新卖出那一个带来的收入，红色是前面几个因为降价少收的钱，两者相减就是 MR。

卖 Q = 4 个

Profit Maximization 垄断怎样定产量和价格（CED PRD-3.B.6）

规则 产量：MR = MC　→　价格：从需求曲线上读 规则和完全竞争企业一样是 MR = MC，不同的是垄断企业的 MR 在需求下方，所以定出来的价格 P > MC。
WORKED EXAMPLE 3冰淇淋车卖几个、卖多少钱？ 原创

The only ice cream truck on an island faces the demand schedule in the table above (P = 11 − Q). Its marginal cost is constant at $3 per cone, and its fixed cost is $5. (a) Find the profit-maximizing quantity and price. (b) Calculate the maximum profit. (c) Would a perfectly competitive market with the same costs produce more or less?

(a) MR 依次为 10、8、6、4、2……只要 MR ≥ MC = $3 就多卖：第 4 个 MR = $4 ≥ 3，第 5 个 MR = $2 < 3 → Q = 4。价格从需求表读：P = $7（不是 MR 的 $4）。
(b) TR = 7 × 4 = 28；TC = 5 + 3 × 4 = 17；利润 = $11。
(c) 完全竞争会生产到 P = MC：P = $3 时需求量 Q = 8 → 产量更多、价格更低。垄断少卖了第 5–8 个，这些冰淇淋顾客愿付的价格（6、5、4、3）都 ≥ MC，却没有被生产——这就是无谓损失。
Quantity (Q)Price, Cost ($)0③ 利润DMRMCATC$16$11$8Qm = 4① MR = MC② 往上到 D
垄断企业三步走：① 在 MR = MC 处定产量 Qm = 4；② 从 Qm 往上走到需求曲线读出价格 Pm = $16（不是 MR = MC 那个点的高度 $8！）；③ 在 Qm 处找到 ATC = $11，利润 = (16 − 11) × 4 = $20。
易错 TRAP
  • 价格不是 MR = MC 交点的高度，而是从 Qm 往上到需求曲线的高度。
  • 垄断企业不会“想定多高就定多高”：价格越高卖得越少，它要的是利润最大，不是价格最高。
  • 利润最大 ≠ TR 最大：TR 最大在 MR = 0 处（Q = 6），利润最大在 MR = MC 处（Q = 4）。
Quantity (Q)Price, Cost ($)0LossDMRMCATCATCPmQm
垄断也可能亏损：需求太弱，在 MR = MC 处 ATC 仍高于价格（红色）。垄断只保证“能自己定价”，不保证赚钱。短期只要 P ≥ AVC 就继续生产（规则和 Unit 3 一样），长期一直亏损就会退出。

练习 Practice 4.2（一）· MR 与定价

以下为历年 AP 真题（带图的题按原题重绘），点选项即时判分并显示解析。

4.2-12000 AP Micro Q10
Which of the following statements is true for a monopolist at the profit-maximizing output level?
  • Price exceeds marginal revenue.
  • Marginal cost exceeds price.
  • Demand is price inelastic.
  • Price equals marginal cost, which equals average total cost.
  • The demand curve intersects the supply curve.
垄断企业 MR < P，在 MR = MC 处生产 → P > MR。(B) 说反了；(C) 它在弹性区生产；(D) 只有完全竞争长期均衡才这样；(E) 垄断没有供给曲线。
4.2-22005 AP Micro Q53
A single-price monopolist's marginal revenue is
  • equal to its price
  • less than its price
  • greater than its price
  • negative when it maximizes revenues
  • zero when it maximizes profit
多卖一单位要把所有单位的价格降下来 → MR < P。(D) 收益最大时 MR = 0；(E) 利润最大时 MR = MC，不一定为 0。
4.2-32005 AP Micro Q55
Which of the following is true if a monopolist's marginal revenue is negative at the current level of output?
  • Demand for its product is unit elastic.
  • Demand for its product is price elastic.
  • Demand for its product is price inelastic.
  • Marginal cost is equal to price.
  • Marginal revenue is equal to price.
MR < 0 → 多卖（降价）反而让 TR 下降 → 需求缺乏弹性。
4.2-42000 AP Micro Q47
Which of the following is true in the elastic range of a firm's demand curve?
  • The firm should expand output to increase economic profits.
  • An increase in price will also lead to an increase in total cost.
  • A decrease in price will likely lead to an increase in total revenue.
  • Marginal revenue is negative.
  • The firm is maximizing total revenue.
弹性区：降价时数量增加的百分比大于价格下降的百分比 → TR 上升（MR > 0）。(A) 是否扩产要比较 MR 与 MC；(D) 弹性区 MR > 0；(E) TR 最大在单位弹性处。
4.2-52021 AP Micro 国际卷 Q32
As the quantity of a good increases, which of the following is true in the elastic region of a monopolist’s demand curve?
  • Marginal revenue and total revenue are negative.
  • Marginal revenue is increasing, and total revenue is positive.
  • Marginal revenue is decreasing, and total revenue is negative.
  • Marginal revenue is positive, and total revenue is increasing.
  • Marginal revenue is negative, and average revenue is decreasing.
弹性区 MR > 0，多卖 TR 增加（MR 在下降，但仍为正）。
4.2-62010 AP Micro Q50
Assume that a monopolist is producing in the inelastic portion of its demand curve. Which of the following will occur if the monopolist decreases its price?
  • Marginal revenue will decrease, but profits will increase.
  • Marginal revenue will increase, but profits will decrease.
  • Total revenue will decrease, but profits will increase.
  • Both total revenue and profits will decrease.
  • Both total revenue and profits will increase.
缺乏弹性区降价 → TR 下降；同时多卖要多花成本 → 利润也下降。所以垄断企业绝不会停留在缺乏弹性区。
4.2-72014 AP Micro 练习卷 Q26
For an unregulated monopolist, the profit-maximizing quantity will always be
  • in the elastic region of the demand curve
  • where marginal revenue equals price
  • where price equals average total cost
  • where price equals marginal cost
  • where the marginal cost curve intersects the demand curve
MC > 0，所以 MR = MC 一定落在 MR > 0 处 → 需求的弹性区。(B)(D)(E) 是完全竞争的条件；(C) 是零经济利润。
4.2-82024 AP Micro Q51
QuantityPrice ($)
170
260
350
440
530
The table provided shows a demand schedule for a good that a firm produces. If the firm's marginal cost to produce this good is constant at $15, what is the firm's profit-maximizing quantity?
  • 1 unit
  • 2 units
  • 3 units
  • 4 units
  • 5 units
TR 依次为 70、120、150、160、150 → MR = 70、50、30、10、−10。MR ≥ MC = $15 做到第 3 个；第 4 个的 MR = $10 < 15 → 3 个。
4.2-92024 AP Micro Q19
Marginal revenue$10
Marginal cost$10
Price$30
Average variable cost$5
Average fixed cost$3
Quantity of output produced10 units
The table provided shows cost and revenue information for Sigmoid, a profit-maximizing monopolist that produces and sells rocket toys. If Sigmoid's average fixed cost increases to $4 because of a change in rent, then its economic profit will
  • remain the same
  • decrease by $1
  • decrease by $10
  • decrease by $20
  • decrease by $30
AFC 从 $3 升到 $4、产量 10 → TFC 增加 $10。固定成本不影响 MR = MC，产量不变 → 利润减少 $10。
4.2-102012 AP Micro Q8
Questions 8-9 refer to the graph below.
QUANTITYPRICE0DemandMarginal RevenueMarginal CostAverage Total Cost10$10$15$20
按原题重绘
If the firm produces 10 units of output, its economic profits will equal
  • 0
  • $50
  • $100
  • $150
  • $200
MR = MC 在 Q = 10；价格读需求 = $20；ATC = $15 → 利润 = (20 − 15) × 10 = $50。
4.2-112012 AP Micro Q9
Questions 8-9 refer to the graph below.
QUANTITYPRICE0DemandMarginal RevenueMarginal CostAverage Total Cost10$10$15$20
按原题重绘
Which of the following is most likely to occur if the firm increases production beyond 10 units?
  • Consumers would be willing to purchase more than 10 units at the price of $20 per unit.
  • The firm would definitely experience a loss.
  • The firm would have to lower its price to sell more than 10 units.
  • The firm’s average cost of production would initially increase.
  • The firm’s profits would increase.
需求向下倾斜：要卖超过 10 个必须降价。(A) 价格 $20 时只卖得出 10 个；(E) 超过 MR = MC 的产量后利润会减少。
4.2-122018 AP Micro 练习卷 Q53
Questions 53-54 refer to the graph of a profit-maximizing firm below.
QuantityPrice0DemandMRMarginal CostAverage Total CostQ₁Q₂Q₃Q₄P₁P₂P₃JKLNM
按原题重绘
The firm’s total revenue is equal to
  • area P1P2KL
  • area P1P3JL
  • area 0P3JQ1
  • area 0P2KQ1
  • area JLM
TR = P × Q：Q1 处价格读需求 J 点 = P3 → 矩形 0P3JQ1。
4.2-132018 AP Micro 练习卷 Q54
Questions 53-54 refer to the graph of a profit-maximizing firm below.
QuantityPrice0DemandMRMarginal CostAverage Total CostQ₁Q₂Q₃Q₄P₁P₂P₃JKLNM
按原题重绘
The firm’s economic profit is equal to
  • area 0P3JQ1
  • area 0P2KQ1
  • area P2P3JK
  • area P1P3JL
  • zero
利润 = (P − ATC) × Q = (P3 − P2) × Q1 → P2P3JK。
4.2-142016 AP Micro 练习卷 Q25
Questions 25-26 refer to the monopoly graph below, where MC = marginal cost, ATC = average total cost, D = demand, and MR = marginal revenue.
QUANTITYPRICE0DMRMCATCP₄Q₁P₃Q₂P₂Q₃P₁
按原题重绘
The profit-maximizing combination of output and price for a single-price monopoly is
  • Q1 and P1
  • Q1 and P2
  • Q1 and P4
  • Q2 and P3
  • Q3 and P2
MR = MC 在 Q1；从 Q1 往上到需求，价格是 P4。P1 只是 MR = MC 交点的高度，不是价格。

Monopoly vs. Perfect Competition 效率与无谓损失

同一个需求、同一套成本，如果由一家垄断企业经营，和由很多完全竞争企业经营，结果有什么不同？

Quantity (Q)Price, Cost ($)0CSPSDS = MC$12Qc = 6
如果这是一个完全竞争行业：MC 就是行业供给曲线，在 D = MC 处 Q = 6、P = $12。CS = PS = ½ × 12 × 6 = 36，总剩余 72，没有无谓损失。
Quantity (Q)Price, Cost ($)0CSPSDWLDMRMC$16Qm = 4$8
垄断：在 Q = 4 生产、定价 $16。CS = ½ × (24 − 16) × 4 = 16；PS = 48；Q = 4 到 6 之间的红色三角形 = DWL = ½ × (16 − 8) × 2 = 8。
完全竞争垄断变化
产量 Q64↓
价格 P$12$16↑
消费者剩余 CS3616↓ 20
生产者剩余 PS3648↑ 12（从消费者那里转来 16，自己少了 4）
总剩余7264↓ 8 = DWL
WORKED EXAMPLE 4用字母读垄断图（AP 选择题考法） 原创

The graph shows the demand, marginal revenue, marginal cost, and average total cost curves for an unregulated monopolist. Using the labels, identify (a) the profit-maximizing quantity and price, (b) the area of economic profit, (c) the area of consumer surplus, (d) the area of deadweight loss, and (e) the price and quantity if this market were perfectly competitive with the same costs.

Quantity (Q)Price, Cost ($)0DMRMCATCP₄Q₁P₃Q₂P₂P₁ABCEF
用字母表示面积：A 是需求的纵截距，B 在需求上，C 是需求与 MC 的交点，E 是 MR = MC，F 在 ATC 上。
(a) MR = MC 在 E 点 → 产量 Q1；往上到需求 B 点 → 价格 P4。
(b) 利润 = (P4 − P2) × Q1 → 矩形 P2P4BF。
(c) 消费者剩余 = 价格 P4 以上、需求以下 → 三角形 AP4B。
(d) 从 Q1 到 D = MC 交点 Q2，需求与 MC 之间 → 三角形 BCE。
(e) 完全竞争在 D = MC（C 点）：价格 P3，产量 Q2。
口诀：产量看 MR = MC，价格看需求，利润看 ATC，DWL 看 D 与 MC 的交点。
垄断的三个“没效率”
  • 配置无效率：P（$16）> MC（$8），产量少于社会最优，存在 DWL。
  • 生产无效率：不在 ATC 最低点生产（Q = 4 时 ATC = $11，而 ATC 最低约 $10.6，在 Q ≈ 5.3）。
  • 剩余转移：一部分消费者剩余变成了垄断利润。由于进入壁垒，这份利润在长期也能保持。

Taxes on a Monopolist 对垄断企业征税

Quantity (Q)Price, Cost ($)0DMRMC₁MC₂ = MC₁ + $6$164$183
每单位征税 $6：MC 上移 $6，MR = MC 的产量从 4 降到 3，价格从 $16 升到 $18——价格只涨了 $2，另外 $4 由企业承担。定额税（lump-sum tax）只提高 ATC，不改变 MC 和 MR，所以产量和价格都不变，只是利润减少。
WORKED EXAMPLE 5征税之后：两种税，两种结果 原创

Use the monopoly graph above (P = 24 − 2Q, MC = 2Q, ATC = Q + 28/Q; currently Q = 4, P = $16, profit = $20). (a) The government imposes a lump-sum tax of $12. What happens to quantity, price, and profit? (b) Instead, the government imposes a per-unit tax of $6. Find the new quantity and price.

(a) 定额税是固定成本：MC、MR 都不变 → Q = 4、P = $16 不变；利润 = 20 − 12 = $8。
(b) 每单位税让 MC 变成 2Q + 6。MR = MC：24 − 4Q = 2Q + 6 → Q = 3，价格读需求 P = 24 − 6 = $18。
和完全竞争企业比：结论完全一样——固定成本变化不影响产量，每单位成本变化会减少产量（Unit 3 下册 3.6）。
LAB 3垄断企业实验室 · Monopoly Decisions

需求 P = 24 − 2Q，MC = 2Q，ATC = Q + 28/Q。拖动产量，看价格、利润和 MR、MC 的比较；再试试两种税。

产量 Q = 4

Natural Monopoly 自然垄断与管制

Quantity (Q)Price, Cost ($)0DMRATCMC$716$108一家做 16 个：ATC = $7两家各做 8 个：ATC = $10
自然垄断：在整个需求范围内 ATC 都在下降（规模经济，CED PRD-3.B.7）。全市需要 16 个单位时，一家企业来做每单位成本 $7；如果两家平分，每家只做 8 个，每单位成本 $10——一家做最便宜。例：地铁、电网、自来水管网。
WORKED EXAMPLE 6城市地铁的票价怎么定？ 原创

A city's subway system is a natural monopoly. Demand is P = 20 − Q (Q in millions of rides per month), marginal cost is constant at $4 per ride, and fixed cost is $48 million per month, so ATC = 4 + 48/Q. Find the quantity, price, economic profit, and deadweight loss (a) if the subway is unregulated, (b) if the city sets P = MC, (c) if the city sets P = ATC.

(a) MR = 20 − 2Q = 4 → Q = 8，P = $12；ATC = 4 + 6 = $10，利润 = (12 − 10) × 8 = $16 million；DWL = ½ × (12 − 4) × (16 − 8) = 32。
(b) P = MC = $4 → Q = 16；ATC = 4 + 3 = $7，利润 = (4 − 7) × 16 = −$48 million（需要补贴 $48 million），DWL = 0。
(c) P = ATC：20 − Q = 4 + 48/Q → Q² − 16Q + 48 = 0 → Q = 12（取较大的根），P = $8，利润 = 0；DWL = ½ × (8 − 4) × (16 − 12) = 8。
下面的三步图就是这三种情况，点按钮切换。

自然垄断拆成几家反而成本更高，所以政府通常不拆分它，而是管制它的价格。点下面三个按钮，比较三种定价：

Quantity (Q)Price, Cost ($)0ProfitDWLDMRATCMC$128$10
① 不管制（unregulated）：企业自己在 MR = MC 处生产：Q = 8，价格读需求 P = $12。ATC = $10，利润 = (12 − 10) × 8 = $16。P > MC，有无谓损失 ½ × (12 − 4) × (16 − 8) = 32。
Quantity (Q)Price, Cost ($)0Loss = 补贴DMRATCMC$716$4
② 社会最优定价 P = MC：政府规定价格 = MC = $4：Q = 16，达到配置效率，没有 DWL。但 ATC = $7 > P，企业每单位亏 $3，共亏 $48（正好等于固定成本）——不补贴企业就会退出。
Quantity (Q)Price, Cost ($)0DWLDMRATCMC$812
③ 公平回报定价 P = ATC：政府规定价格 = ATC（需求与 ATC 的交点）：Q = 12，P = $8，经济利润 = 0，企业愿意留下。比不管制时产量多、价格低，DWL 缩小到 ½ × (8 − 4) × (16 − 12) = 8。
QP经济利润DWL问题
不管制（MR = MC）8$12+$1632产量太少、价格太高
社会最优（P = MC）16$4−$480企业亏损，需要政府补贴
公平回报（P = ATC）12$808仍有少量 DWL，但企业能活下去
FRQ 常考问法 “If the government sets the price to achieve the allocatively efficient quantity (socially optimal price)…” → 找 P = MC； “…so that the firm earns zero economic profit (fair-return price)…” → 找 P = ATC。自然垄断图的标志：ATC 在需求范围内一直下降，MC 在 ATC 下方。

练习 Practice 4.2（二）· 效率、征税与自然垄断

以下为历年 AP 真题（带图的题按原题重绘），点选项即时判分并显示解析。

4.2-152000 AP Micro Q24
If a perfectly competitive industry were monopolized without any changes in cost conditions, the price and quantity produced would change in which of the following ways?
  • Price: Increase；Quantity: Increase
  • Price: Increase；Quantity: Decrease
  • Price: Increase；Quantity: May increase or decrease
  • Price: Decrease；Quantity: Increase
  • Price: Decrease；Quantity: Decrease
垄断在 MR = MC 处生产，产量少于完全竞争在 D = MC 处的产量，价格更高 → 价格上升、产量减少。
4.2-162013 AP Micro Q44
A monopolist is inefficient from society’s point of view because
  • it overproduces output and charges a price above marginal cost
  • it underproduces output and charges a price above marginal cost
  • it sometimes colludes with other firms to obtain higher prices
  • the price the monopolist charges is too low and it does not advertise
  • consumers must pay whatever price the monopolist charges
产量太少、P > MC → 存在无谓损失。
4.2-172021 AP Micro 国际卷 Q35
A monopoly is producing the allocatively efficient output if, for the last unit produced,
  • price equals minimum average total cost
  • price equals marginal cost
  • marginal cost equals marginal revenue
  • marginal cost equals average variable cost
  • marginal revenue equals average variable cost
配置效率 = P = MC：消费者对最后一单位的边际收益等于生产它的边际成本。(A) 是生产效率；(C) 是利润最大化条件。
4.2-182014 AP Micro 练习卷 Q8
Compared to a perfectly competitive industry with the same demand and cost curves, a monopoly’s price and output will be which of the following?
  • Price: The same；Output: Higher
  • Price: Higher；Output: The same
  • Price: Higher；Output: Lower
  • Price: Lower；Output: The same
  • Price: Lower；Output: Higher
与完全竞争相比，垄断价格更高、产量更低。
4.2-192013 AP Micro Q29
If the government imposes a per-unit tax on the output of a monopoly with a downward-sloping demand curve, the burden of the tax will be
  • borne entirely by consumers
  • borne entirely by the monopolist
  • borne entirely by the government
  • shared by the monopolist and the government
  • shared by consumers and the monopolist
每单位税使 MC 上移，垄断减产、提价，但价格上涨幅度小于税额 → 由消费者和垄断企业共同负担。
4.2-202014 AP Micro 练习卷 Q27
If a per-unit tax is imposed on a monopolist, how will the monopolist’s marginal cost curve, output, and the price paid by consumers be affected?
  • Marginal Cost: Shift down；Output: Increase；Price: Decrease
  • Marginal Cost: Shift down；Output: Decrease；Price: Decrease
  • Marginal Cost: No shift；Output: Decrease；Price: Decrease
  • Marginal Cost: Shift up；Output: Decrease；Price: Increase
  • Marginal Cost: Shift up；Output: Increase；Price: Increase
每单位税是可变成本 → MC 上移 → MR = MC 的产量减少 → 价格（读需求）上升。
4.2-212022 AP Micro 国际卷 Q60
Prosper Company is a profit-maximizing monopoly producing toy cubes. Prosper Company is currently earning positive economic profit. Which of the following combinations of events will definitely cause Prosper Company’s profit to increase?
  • A decrease in demand for toy cubes and a decrease in fixed costs to produce toy cubes
  • A decrease in demand for toy cubes and an increase in fixed costs to produce toy cubes
  • A decrease in demand for toy cubes and a decrease in variable costs to produce toy cubes
  • An increase in demand for toy cubes and an increase in government subsidies to produce toy cubes
  • An increase in demand for toy cubes and an increase in fixed costs to produce toy cubes
需求增加 → MR 右移，产量、价格、利润都增加；补贴降低成本 → 利润增加。两者都让利润上升，所以一定增加。(C) 需求下降与可变成本下降方向相反，不确定。
4.2-222010 AP Micro Q54
Questions 54-55 are based on the graph below, which shows the cost and revenue curves of a monopoly firm.
QUANTITYPRICE0DemandMarginal RevenueMarginal CostAverage Total CostQ₁Q₂Q₃Q₄RSTUVIJKMLN
按原题重绘
The economic profit of the profit-maximizing monopolist is given by the area
  • RSJI
  • R0Q1I
  • RULI
  • RVNI
  • U0Q4M
MR = MC 在 Q1（N 点）；价格读需求 = R（I 点）；Q1 处 ATC = U（L 点）→ 利润 = (R − U) × Q1 = RULI。
4.2-232010 AP Micro Q55
Questions 54-55 are based on the graph below, which shows the cost and revenue curves of a monopoly firm.
QUANTITYPRICE0DemandMarginal RevenueMarginal CostAverage Total CostQ₁Q₂Q₃Q₄RSTUVIJKMLN
按原题重绘
If this were a perfectly competitive industry with the same costs as shown on the graph, the equilibrium price and output would be which of the following?
  • Price: 0R；Output: Q1
  • Price: 0S；Output: Q2
  • Price: 0T；Output: Q3
  • Price: 0U；Output: Q1
  • Price: 0U；Output: Q4
完全竞争行业在 D = MC（K 点）处：价格 0T，产量 Q3。
4.2-242024 AP Micro Q34
QuantityPrice, Cost ($)0DemandMRMarginal CostGHFEABCD
按原题重绘
What is the area of consumer surplus in the graph provided when the firm maximizes its profit?
  • ABG
  • ACF
  • ADE
  • BDC
  • 0GBH
利润最大在 MR = MC（D 点）→ 产量 H，价格读需求 = G（B 点）。CS = 需求以下、价格 G 以上的三角形 ABG。
4.2-252017 AP Micro 练习卷 Q50
QUANTITYPRICE0DemandMRMarginal CostGQHFKIJ
按原题重绘
In the diagram above, the deadweight loss from a profit-maximizing monopolist is represented by area
  • FGK
  • FHI
  • IJK
  • GHIK
  • 0HIQ
DWL = 从 Q 到 D = MC 交点（J）之间、需求与 MC 围成的三角形 IJK。
4.2-262010 AP Micro Q30
If government regulators set price such that a natural monopolist earns only normal profits, price will be set equal to
  • marginal revenue
  • marginal cost
  • average total cost
  • average revenue
  • average variable cost
只赚正常利润 = 经济利润为 0 → P = ATC（公平回报定价）。
4.2-272022 AP Micro 国际卷 Q37
Regulating a natural monopoly to set its output price equal to the marginal cost of production will result in economic losses because for the last unit of output produced, marginal cost is
  • less than the marginal willingness to pay and less than average total cost
  • greater than the marginal willingness to pay and less than average total cost
  • equal to the marginal willingness to pay and less than average total cost
  • equal to the marginal willingness to pay and equal to average total cost
  • greater than the marginal willingness to pay and equal to average total cost
P = MC 时，价格就是最后一单位的边际愿付，所以 MC = 边际愿付；自然垄断的 MC 在 ATC 下方 → P = MC < ATC → 亏损。
4.2-282023 AP Micro 国际卷 Q33
A natural monopoly exists when a single firm
  • produces a quantity of output that maximizes the total producer surplus
  • produces the allocatively efficient quantity of output of a good
  • has an upward-sloping marginal cost curve
  • has a downward-sloping average total cost curve over the relevant range of output
  • sets price equal to marginal cost in order to maximize profit
自然垄断：在相关产量范围内（整个市场需求）ATC 一直下降，一家企业来生产成本最低。
4.2-292017 AP Micro 练习卷 Q36
QUANTITYPRICE0DemandMRAverage Total CostMarginal CostP₄Q₁P₂Q₂P₃Q₄P₁Q₅Q₃
按原题重绘
Questions 36-37 refer to the following diagram of a natural monopolist.

The firm shown in the diagram above qualifies as a natural monopoly because
  • the demand curve is downward sloping
  • the demand curve lies above the marginal revenue curve
  • the average total cost is decreasing in the relevant range of market demand
  • the firm can maximize profit with any output level it chooses
  • marginal revenue is positive at the profit-maximizing output level
在整个市场需求范围内 ATC 都在下降 → 一家企业生产成本最低，这就是自然垄断。
4.2-302017 AP Micro 练习卷 Q37
QUANTITYPRICE0DemandMRAverage Total CostMarginal CostP₄Q₁P₂Q₂P₃Q₄P₁Q₅Q₃
按原题重绘
Questions 36-37 refer to the following diagram of a natural monopolist.

If government regulated the natural monopoly to produce the output resulting in zero economic profit, then the output would be
  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
零经济利润 → P = ATC → 需求与 ATC 的交点 Q4（价格 P3）。Q2 是不管制产量，Q5 是 P = MC 的产量。

4.3Price Discrimination 价格歧视

定义 DEFINITION（CED PRD-3.B.8） Price discrimination：同一种商品，向不同顾客收取不同价格，而且价格差别不是因为成本不同。有市场势力的企业这样做，是为了把一部分消费者剩余变成自己的利润。
三个条件
  • 有市场势力：价格接受者（完全竞争企业）无法这样做。
  • 能区分顾客：知道或能分辨谁愿意付更多（需求弹性不同）。
  • 能防止转卖：否则低价买到的人会转卖给高价顾客。

Everyday Examples 身边的价格歧视

🎟️
学生票、老人票：用学生证、身份证区分顾客；这两类人收入较低、需求更有弹性，所以便宜。
✈️
机票提前买便宜：临近起飞才买的多是商务客，需求缺乏弹性；机票实名，不能转卖。
🎬
电影早场便宜：愿意上午去看的人时间灵活、对价格敏感。
💻
软件教育版：凭学校邮箱购买，学生预算有限；授权绑定账号，防止转卖。
🧾
优惠券：愿意花时间领券的人更在乎价格，企业借此“筛选”出价格敏感的顾客。
易错 TRAP · 不是所有价格差都叫价格歧视 偏远地区运费更贵、因此售价更高，是成本不同造成的，不算价格歧视。价格歧视是同样的成本、不同的价格。
WORKED EXAMPLE 7商务客和度假客，谁的机票更贵？ 原创

An airline with market power sells seats on the same flight to two groups: business travelers, who must fly on specific dates, and vacationers, who can choose among many dates and destinations. (a) Which group has the more elastic demand? (b) Which group will be charged the higher price? (c) Give one way the airline separates the two groups, and explain why resale must be prevented.

(a) 度假客可以换日期、换目的地，替代品多 → 需求更有弹性；商务客必须在那天到那个城市 → 需求缺乏弹性。
(b) 商务客付更高的价格。规律：需求越缺乏弹性的群体，价格越高；需求越有弹性的群体，价格越低（涨价会流失很多人）。
(c) 例如：提前三周买便宜、临近起飞买贵；要求周六晚上在目的地过夜才能买到低价票。机票实名、不能转让，否则度假客会低价买票转卖给商务客，价格歧视就失效了。

Perfect Price Discrimination 完全价格歧视（CED PRD-3.B.9）

最极端的情况：企业知道每位顾客愿意付的最高价，并且就收这么多（也叫 first-degree price discrimination）。

WORKED EXAMPLE 8小镇唯一的修车师傅 原创

The only mechanic in a small town has five customers, each wanting one repair. Their maximum willingness to pay is A $10, B $8, C $6, D $4, E $2. The marginal cost of a repair is $3. (a) If the mechanic must charge everyone the same price, what price maximizes profit? Calculate profit, consumer surplus, and deadweight loss. (b) If the mechanic can perfectly price discriminate, calculate profit, consumer surplus, and deadweight loss.

单一价格谁买利润 = (P − 3) × 人数
$10A7
$8A、B10
$6A、B、C9
$4A–D4
(a) 价格 $8 利润最大（$10）。CS = A 多出的 10 − 8 = $2。C、D 愿付 $6、$4，都高于 MC $3，却没被服务：DWL = (6 − 3) + (4 − 3) = $4。
(b) 每人收 WTP：A–D 都修（E 的 $2 < MC，不修）。利润 = 7 + 5 + 3 + 1 = $16，CS = 0，DWL = 0。
结论：价格歧视让利润从 $10 增加到 $16，其中 $2 来自原来 A 的消费者剩余，$4 来自原来的无谓损失。
$12$10顾客 A$8顾客 B$6顾客 C$4顾客 D$2顾客 EMC = $3P = $8
单一价格 $8：只有 A、B 买。蓝色 = 消费者剩余（A 多出的 $2）；绿色 = 利润（每个 $8 − $3，共 $10）；红色 = C、D 愿付的钱（$6、$4）高于 MC，却因为价格太高没买——无谓损失 3 + 1 = $4。
$12收 $10$10顾客 A收 $8$8顾客 B收 $6$6顾客 C收 $4$4顾客 D$2顾客 EMC = $3
完全价格歧视：每人收他愿付的最高价。A–D 都买（E 愿付 $2 < MC，不卖），利润 = 7 + 5 + 3 + 1 = $16，消费者剩余 = 0，没有无谓损失——产量和完全竞争一样，但所有剩余都归企业。
Quantity (Q)Price, Cost ($)0利润 = 72D = MRMC$12Q = 6每个单位按买家愿付的最高价卖 → 需求线就是 MR
用 4.2 的同一个市场：完全价格歧视的企业每多卖一个，只降这一个的价格，前面的不用降，所以 MR = D。它会一直卖到 D = MC（Q = 6），和完全竞争的产量一样；D 与 MC 之间的整块三角形（72）全部变成企业的利润。
完全竞争单一价格垄断完全价格歧视
产量 Q646
消费者剩余 CS36160
生产者剩余 PS364872
DWL080
配置效率？是否（P > MC）是（最后一单位 P = MC）
完全价格歧视的结论
  • MR = D：多卖一个只降这一个的价格，所以需求曲线就是边际收益曲线。
  • 产量在 P = MC 处，和完全竞争一样 → 达到配置效率，没有 DWL。
  • 消费者剩余 = 0，全部经济剩余都变成企业的利润。
  • “有效率”不等于“对消费者好”：总剩余最大，但全被企业拿走了。
LAB 4一个价格，还是每人一个价格？ · Price Discrimination

10 位顾客各买一个，柱子高度是愿意支付的最高价（WTP）。先用单一价格找利润最大的价格，再切换到“完全价格歧视”比较。

单一价格 P = $14
边际成本 MC = $6

练习 Practice 4.3

以下为历年 AP 真题（带图的题按原题重绘），点选项即时判分并显示解析。

4.3-12013 AP Micro Q55
Which of the following is necessary for a firm to practice price discrimination?
  • The government strictly enforces antitrust laws.
  • The firm is a member of a cartel.
  • The firm can prevent resale of its goods.
  • The demand curve for the product is perfectly inelastic.
  • The demand curve for the product is perfectly elastic.
必须能防止转卖，否则低价买到的人会转卖给高价顾客。(D)(E) 价格歧视需要不同顾客的弹性不同，而不是完全无弹性或完全有弹性。
4.3-22014 AP Micro 练习卷 Q38
In order for a firm to engage in price discrimination, it must be
  • able to separate consumers into different groups based on demand elasticities
  • producing in the inelastic portion of its demand curve to raise its price and increase total revenue
  • a price taker
  • experiencing economies of scale in the relevant range of production
  • experiencing constant marginal cost
要能按需求弹性把顾客分成不同群体。(C) 价格接受者无法价格歧视。
4.3-32018 AP Micro 练习卷 Q13
Which of the following is a necessary condition for price discrimination?
  • Resale of the product or service is possible.
  • Buyers have identical elasticities of demand.
  • The seller can separate consumers according to their elasticities of demand.
  • The seller is a perfectly competitive firm.
  • Buyers are aware of prices charged to other buyers.
卖家能按需求弹性区分顾客。(A) 应是不能转卖；(B) 弹性必须不同；(D) 必须有市场势力。
4.3-42016 AP Micro 练习卷 Q40
A firm with market power engages in price discrimination to
  • earn a higher profit
  • increase consumer surplus
  • decrease deadweight loss
  • make its demand more elastic
  • make its demand more inelastic
价格歧视的目的：把消费者剩余变成利润，提高利润。消费者剩余会减少；需求弹性不会因此改变。
4.3-52019 AP Micro 练习卷 Q23
If a firm engages in perfect price discrimination, it charges
  • each customer the highest price the customer is willing to pay
  • each customer the average cost of the product
  • each customer the lowest price the customer is willing to pay
  • different prices to customers based on how old they are
  • different prices to customers based on how many units of output they buy
完全价格歧视 = 每位顾客付自己愿付的最高价。(D)(E) 是按群体或按数量的不完全价格歧视。
4.3-62010 AP Micro Q38
Assume that a profit-maximizing monopoly is charging a single price. If the monopoly can price discriminate and charge each consumer what he or she is willing to pay, which of the following will occur?
  • The quantity of output produced will increase.
  • Total cost will decrease.
  • Economic profit will decrease.
  • Consumer surplus will increase.
  • Demand will decrease.
完全价格歧视时 MR = D，会一直生产到 D = MC → 产量增加。利润增加（C 错），消费者剩余减少（D 错）。
4.3-72022 AP Micro 国际卷 Q3
Which of the following is true for a price-discriminating firm?
  • The firm charges different prices to different consumers for the same product.
  • The firm charges different prices for different products.
  • The firm pays more per unit of labor than it pays per unit of capital.
  • The firm pays more per unit of capital than it pays per unit of labor.
  • The firm sells different quantities to its consumers but charges each of them the same price.
价格歧视：同一种商品向不同顾客收不同价格。
4.3-82022 AP Micro 国际卷 Q44
If a monopolist can increase output and profit by engaging in perfect price discrimination, allocative efficiency will
  • improve even though consumer surplus goes to zero
  • possibly improve or get worse depending on whether consumer surplus increases or decreases
  • possibly improve or get worse depending on whether consumer surplus increases by more or less than the increase in producer surplus
  • get worse because producer surplus increases by more than the increase in consumer surplus
  • be unchanged because although producer surplus increases, total surplus is unaffected
产量增加到 P = MC，DWL 消失 → 配置效率改善；但消费者剩余变成 0。
4.3-92025 AP Micro 国际卷 Q12
A firm is selling its product in two separate markets. If the firm practices price discrimination, it is most likely to charge a higher price in the market that
  • has lower marginal cost
  • has no barriers to entry
  • contains a binding price ceiling
  • has fewer substitutes available
  • allows resale of the product
替代品少 → 需求缺乏弹性 → 收更高的价格。
4.3-102005 AP Micro Q35
The price of an airline ticket is typically lower if a traveler buys the ticket several weeks before the flight's departure date rather than on the day of departure. This pricing strategy is based on the assumption that
  • travelers are not aware of how airline prices change across time
  • travelers do not have alternative modes of transportation
  • travelers will pay any price to travel as the departure date approaches
  • the marginal cost of the last few seats on an airplane is higher than that for the first few seats
  • travelers' demand becomes less elastic as the departure date approaches
临近起飞才买票的多是必须出行的商务客，需求缺乏弹性 → 收更高价（按时间区分顾客的价格歧视）。(D) 成本差别不是原因。
4.3-112021 AP Micro 国际卷 Q43
Price ($)Quantity demanded
19
28
37
46
55
64
73
82
91
A profit-maximizing monopolist faces the market demand schedule provided and has a constant marginal cost of $5. If the monopolist engages in perfect price discrimination, its total revenue will equal which of the following?
  • $10
  • $15
  • $25
  • $35
  • $45
完全价格歧视：每一单位按买家愿付的价格卖，只要价格 ≥ MC = $5。第 1–5 单位的愿付价格是 9、8、7、6、5 → TR = 9 + 8 + 7 + 6 + 5 = $35。
4.3-122016 AP Micro 练习卷 Q26
Questions 25-26 refer to the monopoly graph below, where MC = marginal cost, ATC = average total cost, D = demand, and MR = marginal revenue.
QUANTITYPRICE0DMRMCATCP₄Q₁P₃Q₂P₂Q₃P₁
按原题重绘
If the monopolist could engage in perfect price discrimination, the monopolist’s total output and the price charged for the last unit of output sold would be
  • Q1 and P1
  • Q1 and P2
  • Q1 and P4
  • Q2 and P3
  • Q3 and P2
完全价格歧视时 MR = D，生产到 D = MC 的 Q2，最后一单位的价格是 P3。
4.3-132021 AP Micro 国际卷 Q42
QuantityPrice, Cost0DMRMCATCQ₀Q₁Q₂Q₃Q₄
按原题重绘
The graph above shows the demand (D), marginal revenue (MR), marginal cost (MC), and average total cost (ATC) curves for a monopoly. Based on the information in the graph provided, what is the profit-maximizing quantity for a monopolist that engages in perfect price discrimination?
  • Q0
  • Q1
  • Q2
  • Q3
  • Q4
完全价格歧视 → 生产到 D = MC → Q3。Q0 是单一价格时的 MR = MC；Q1 是 TR 最大；Q2 是 ATC 最低；Q4 是 D = ATC。

ΣSummary 本册总结与易错点

4.1 Imperfect Competition

不完全竞争 = 垄断、寡头、垄断竞争（以及要素市场的买方垄断）。企业面对向下倾斜的需求，多卖必须降价；P > MC，因而没有效率。进入壁垒（高固定成本、法律壁垒、控制关键资源）让经济利润在长期也能存在。

4.2 Monopoly · 收益

MR < P：多卖一个要降低所有单位的价格。直线需求 P = a − bQ 时 MR = a − 2bQ（同截距、斜率加倍）。MR > 0 是弹性区，MR = 0 时 TR 最大（单位弹性）；垄断只在弹性区生产。

4.2 Monopoly · 决策与效率

MR = MC 定产量，从需求读价格，利润 = (P − ATC) × Q。与完全竞争相比：产量少、价格高、CS 减少、存在 DWL，既无配置效率也无生产效率。定额税不改变 Q 和 P；每单位税使 Q 减少、P 上升。

4.2 Natural Monopoly

ATC 在整个需求范围内下降，一家企业成本最低。不管制：MR = MC；社会最优定价 P = MC（无 DWL，但亏损，需要补贴）；公平回报定价 P = ATC（利润为 0，DWL 变小）。

4.3 Price Discrimination

条件：市场势力、能区分顾客、能防止转卖。需求越缺乏弹性的群体价格越高。完全价格歧视：MR = D，产量在 P = MC 处（与完全竞争相同），没有 DWL，CS = 0，全部剩余归企业。
易错点 TOP 10
  1. 垄断价格在 MR = MC 交点的高度 ✗ → 从 Qm 往上读需求曲线。
  2. 垄断想定多高价就定多高 ✗ → 价格越高卖得越少，它追求的是利润最大。
  3. MR 曲线画在需求上方或与需求平行 ✗ → 同一纵截距、斜率加倍、在需求下方。
  4. 利润最大 = TR 最大 ✗ → TR 最大在 MR = 0 处，利润最大在 MR = MC 处。
  5. 垄断可以在需求缺乏弹性的区域生产 ✗ → 只在富有弹性的区域（MR > 0）。
  6. 垄断一定赚钱 ✗ → 需求太弱时也会亏损；但有进入壁垒时，经济利润可以长期存在。
  7. 定额税会让垄断企业减产、涨价 ✗ → 定额税不影响 MC 和 MR，Q 和 P 都不变。
  8. 自然垄断按 P = MC 定价就皆大欢喜 ✗ → 企业会亏损（ATC > P），需要补贴。
  9. 价格歧视就是不同地方价格不同 ✗ → 成本不同造成的价差不算，必须是同样成本、不同价格。
  10. 完全价格歧视对消费者最好 ✗ → 没有 DWL，但消费者剩余为 0。

✎Unit 4（上）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。

T12005 AP Micro Q25
A monopoly is different from a perfectly competitive firm in that a monopoly
  • does not have a U-shaped average total cost curve
  • has an average fixed cost curve that is perfectly horizontal
  • has a marginal revenue curve that lies below its demand curve
  • always earns economic profits
  • operates in the inelastic segment of its demand curve
垄断的 MR 曲线在需求曲线下方；完全竞争企业 MR = P。(A)(B) 成本曲线形状与市场结构无关；(D) 垄断也可能亏损；(E) 垄断在弹性区生产。
T22015 AP Micro 练习卷 Q8
Which of the following is most likely to occur if a single-price monopolist is replaced by a perfectly competitive market?
  • Prices will increase.
  • The deadweight loss will decrease.
  • Profits will increase.
  • Output will decrease.
  • The firm’s cost curves will shift upward.
完全竞争取代垄断 → 产量增加到 P = MC → DWL 减少；价格下降、利润减少。
T32018 AP Micro 练习卷 Q26
A monopolist’s demand curve is necessarily
  • the same as the market demand curve
  • the same as its marginal revenue curve
  • upward sloping
  • perfectly elastic
  • perfectly inelastic
只有一家企业 → 它的需求就是整个市场的需求（向下倾斜）。
T42019 AP Micro 练习卷 Q38
Which of the following is true when a profit-maximizing monopolist produces in the elastic portion of its demand curve?
  • It can increase total revenue by raising price.
  • It can decrease average total cost by reducing output.
  • Price is equal to marginal revenue.
  • Marginal revenue is less than marginal cost.
  • Marginal revenue is positive.
弹性区 MR > 0。(A) 弹性区提价会让 TR 下降；(C) MR < P；(D) 利润最大时 MR = MC。
T52022 AP Micro 国际卷 Q33
Which of the following is true in the inelastic region of a monopolist’s demand curve as output increases?
  • Marginal revenue and total revenue are negative.
  • Marginal revenue is increasing and total revenue is positive.
  • Marginal revenue is decreasing and total revenue is negative.
  • Marginal revenue is positive and total revenue is increasing.
  • Marginal revenue is negative and total revenue is decreasing.
缺乏弹性区 MR < 0，多卖让 TR 下降（TR 本身仍然为正）。
T62016 AP Micro 练习卷 Q9
A single-price monopolist is currently producing in the inelastic portion of its market demand curve. In order to maximize profits, the monopolist should change the price and output in which of the following ways?
  • Price: Increase；Output: Increase
  • Price: Increase；Output: Decrease
  • Price: Increase；Output: Not change
  • Price: Decrease；Output: Increase
  • Price: Not change；Output: Increase
缺乏弹性区：提价、减产 → TR 上升、成本下降，利润增加；一直调整到弹性区的 MR = MC 为止。
T72024 AP Micro Q58
Task-It Basket is a firm with some market power. The firm produces and sells baskets for $50 each. At the current quantity of baskets, the firm's marginal revenue is $27 and marginal cost is $34. Task-It Basket should do which of the following to maximize its profits?
  • Produce fewer baskets because marginal revenue is less than marginal cost.
  • Produce fewer baskets because marginal revenue is less than price.
  • Produce fewer baskets because price is greater than marginal cost.
  • Produce more baskets because price is greater than marginal cost.
  • Produce more baskets because price is greater than marginal revenue.
MR $27 < MC $34 → 最后一个篮子亏 $7 → 减少产量。(B)(E) 用 P 与 MR 比较没有意义：有市场势力的企业 MR 总是低于 P。
T82025 AP Micro 国际卷 Q25
Which of the following will definitely result in a decrease in a monopoly’s profit-maximizing quantity of output?
  • A decrease in variable costs and the ability to perfectly price discriminate
  • A decrease in variable costs and a decrease in demand for the monopoly’s output
  • An increase in variable costs and a decrease in demand for the monopoly’s output
  • An increase in fixed costs and an increase in demand for the monopoly’s output
  • An increase in fixed costs and the ability to perfectly price discriminate
可变成本上升 → MC 上移 → 产量减少；需求下降 → MR 左移 → 产量减少。两者都使产量减少 → 一定减少。(D) 固定成本不影响产量，需求增加还会使产量增加。
T92016 AP Micro 练习卷 Q15
If the goal of government regulators of a natural monopoly is to reduce deadweight loss without subsidizing the monopolist, government regulators would set a price equal to
  • average variable cost
  • average total cost
  • average fixed cost
  • short-run marginal cost
  • long-run marginal cost
既要减少 DWL 又不补贴 → P = ATC（公平回报定价），企业经济利润为 0。P = MC 会亏损，需要补贴。
T102022 AP Micro 国际卷 Q38
Which of the following actions would a government take when regulating a natural monopoly to produce the socially optimal quantity?
  • Impose a lump-sum tax on the monopolist’s profit.
  • Impose a per-unit tax on the monopolist’s profit.
  • Open the market for competition to lower the monopoly price.
  • Regulate the monopoly to charge a price equal to the average total cost.
  • Grant a lump-sum subsidy to the monopoly.
社会最优产量要求 P = MC，自然垄断会亏损 → 政府给定额补贴弥补亏损。(D) 是公平回报定价，达不到社会最优产量。
T112015 AP Micro 练习卷 Q57
Questions 57-58 refer to the graph of a monopolist shown below.
QUANTITYPRICE0DemandMarginal RevenueMarginal CostAverage Total CostQ₁Q₂Q₃Q₄P₁P₂P₃P₄P₅RSTUVW
按原题重绘
If the monopolist is unregulated, its profit-maximizing price and output level would lead to a deadweight loss equal to area
  • RUV
  • RTV
  • RTW
  • TUV
  • UVW
不管制：MR = MC 在 V（Q1），价格 P5（R 点）；社会最优在 D = MC 的 T 点（Q3）→ DWL = 三角形 RTV。
T122015 AP Micro 练习卷 Q58
Questions 57-58 refer to the graph of a monopolist shown below.
QUANTITYPRICE0DemandMarginal RevenueMarginal CostAverage Total CostQ₁Q₂Q₃Q₄P₁P₂P₃P₄P₅RSTUVW
按原题重绘
If the government regulates the monopolist to set price equal to average total cost, it will establish the price at
  • P5
  • P4
  • P3
  • P2
  • P1
P = ATC → 需求与 ATC 交于 U → 价格 P2。
T132005 AP Micro Q26
Price discrimination occurs when
  • the supply of the product is elastic
  • a product's average cost is greater than its average revenue
  • a product's average cost is less than its average revenue
  • differences in a product's price reflect differences in marginal costs
  • differences in a product's price do not reflect differences in costs of production
价格歧视：价格差异不反映生产成本的差异。(D) 如果价格差异反映 MC 的差异，就不是价格歧视。
T142024 AP Micro Q13
Which of the following will happen if a profit-maximizing firm begins to engage in perfect price discrimination?
  • Total units sold will decrease.
  • All consumers will be better off.
  • Deadweight loss will increase.
  • The firm's economic profit will increase.
  • The firm's marginal cost will decrease.
完全价格歧视把消费者剩余变成利润 → 利润增加。产量增加（A 错），DWL 消失（C 错），消费者剩余为 0（B 错），MC 不变。
T152023 AP Micro 国际卷 Q40
A firm with market power sells its product in two markets. The firm faces the same cost curves in both markets but faces a relatively elastic demand in one market for its product and a relatively inelastic demand in the other market for that same product. Which of the following will increase the firm’s profits?
  • The firm charges a lower price in both markets.
  • The firm charges a higher price in both markets.
  • The firm charges a price equal to its marginal cost in both markets.
  • The firm charges a lower price in the market with elastic demand and a higher price in the market with inelastic demand.
  • The firm charges a lower price in the market with inelastic demand and a higher price in the market with elastic demand.
弹性需求的市场收低价，缺乏弹性的市场收高价 → 利润增加。
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FRQFree-Response Practice 自由问答

三道都是 AP 真题。前两道的评分标准依照 College Board 官方 Scoring Guidelines 整理；2024 年这道没有官方评分标准，评分要点由本讲义整理。先在纸上完整作答（包括画图），再展开对照。

FRQ 1 · 2023 AP Micro FRQ Set 1 Q1（a–c）· 7 分 · 专利垄断 RKB 真题

RKB is a profit-maximizing monopoly that produces a new, patented electronic device. RKB is earning positive economic profit.

(a) Draw a correctly labeled graph for RKB, and show each of the following.
  (i) The profit-maximizing quantity, labeled QM
  (ii) The profit-maximizing price, labeled PM
  (iii) The average total cost curve consistent with RKB earning positive economic profit
  (iv) The area representing the deadweight loss, shaded completely
(b) The government wants RKB to produce the allocatively efficient quantity. Would the government impose a binding price ceiling, a binding price floor, a per-unit tax, or a lump-sum tax?
(c) Suppose that the government does not impose the policy you identified in part (b). Consumers now become aware of research that confirms the use of this new device harms users' vision. Given widespread consumer awareness of this research, what will happen to RKB's profit-maximizing quantity in the short run? Explain.

原题还有 (d) 劳动市场部分（3 分），属于 Unit 5 要素市场，放到 Unit 5 讲义里练习。

Point得分要求与解析
(a) 1 pt正确标注的垄断图：向下倾斜的需求 D，以及在 D 下方、向下倾斜的 MR。
(a) 1 pt画出 MC，在 MR = MC 处标出 QM。
(a) 1 pt从 QM 往上到需求曲线，标出 PM。
(a) 1 ptATC 在 QM 处低于 PM（有经济利润），且 MC 穿过 ATC 的最低点。
(a) 1 pt完整涂出 DWL：QM 到需求与 MC 交点之间、需求与 MC 围成的三角形。
(b) 1 ptBinding price ceiling（有效的价格上限）。把价格限制在需求与 MC 的交点高度，企业面对的 MR 在这一段变成水平线，会把产量增加到配置有效的数量。征税只会让产量更少。
(c) 1 pt产量减少：需求下降，MR 随之左移，与 MC 在更小的产量处相交。
QuantityPrice, Cost0DWLDMRMCATCPMQM
(a) 参考图：MR 在需求下方；QM 在 MR = MC 处；PM 从需求读；ATC 在 QM 处低于 PM，MC 穿过 ATC 最低点；DWL 是 QM 与 D = MC 交点之间、需求与 MC 围成的三角形。
FRQ 2 · 2023 AP Micro FRQ Set 2 Q3 · 5 分 · 自然垄断与管制 真题

The graph shows the cost and revenue curves for an unregulated, profit-maximizing monopoly.

QuantityPrice, Cost0DemandMRAverage Total CostMarginal CostabcdefghjQ₁Q₂Q₃Q₄P₇P₆P₅P₄P₃P₂P₁
按原题重绘

(a) Is the firm shown in this graph a natural monopoly? Explain.
(b) Using the labeling from the graph, identify the area representing the deadweight loss for this profit-maximizing monopoly.
(c) In order to improve resource allocation, the government sets a price that results in the firm earning zero economic profit.
  (i) Using the labeling from the graph, identify the price and resulting quantity the firm would produce.
  (ii) Will this government policy eliminate the deadweight loss? Explain using labeling from the graph.
(d) Instead, the government decides to set a price that results in the socially optimal quantity of output. Will the firm earn positive, negative, or zero economic profit? Explain using labeling from the graph.

Point得分要求与解析
(a) 1 ptYes：在整个有效需求范围内 ATC 都在下降（规模经济贯穿整个市场）。
(b) 1 ptDWL = bfg（或 ½ × (P5 − P1) × (Q4 − Q2)）。不管制时在 MR = MC（f 点）生产 Q2，价格 P5；社会最优产量是 D = MC 的 Q4。
(c)(i) 1 pt价格 P3，产量 Q3（需求与 ATC 的交点 c，P = ATC，经济利润为 0）。
(c)(ii) 1 ptNo：还剩下 DWL = 三角形 cjg（DWL 减少了 bfjc，但没有消失）；在 Q3 处 P3 仍高于 MC，Q3 小于 Q4。
(d) 1 ptNegative：社会最优产量 Q4 处价格 = MC = P1，而 ATC = P2 > P1，亏损面积 = P1P2dg = (P2 − P1) × Q4。
对照 4.2 的自然垄断三步图：这道题的标度正好是 D：P = 20 − Q、MC = 4、ATC = 4 + 48/Q，即 Q2 = 8、Q3 = 12、Q4 = 16，P5 = $12、P3 = $8、P1 = $4。
FRQ 3 · 2024 AP Micro FRQ Set 2 Q1 · 10 分 · Arzeye Pharma 专利药 真题

Arzeye Pharma has a patent, a legal barrier to entry, on its newly developed eye treatment that cures common eye problems. Arzeye Pharma is currently earning positive economic profit and is producing the profit-maximizing quantity of eye treatments.

(a) Draw a correctly labeled graph for Arzeye Pharma and show each of the following.
  (i) The profit-maximizing quantity of eye treatments, labeled Q*
  (ii) The profit-maximizing price, labeled P*
  (iii) The average total cost curve consistent with positive economic profit, labeled ATC
  (iv) The area representing consumer surplus, shaded completely
(b) Suppose Arzeye Pharma wants to charge a price that maximizes its total revenue rather than its profit.
  (i) On your graph in part (a), show the revenue-maximizing quantity, labeled QR.
  (ii) At quantity QR identified in part (b)(i), is the demand for eye treatments elastic, inelastic, or unit elastic?
(c) Suppose now that Arzeye Pharma engages in perfect price discrimination.
  (i) On your graph in part (a), show the lowest price that Arzeye Pharma would charge, labeled P2.
  (ii) What would happen to consumer surplus? Explain.
(d) Suppose instead that Arzeye Pharma's patent expires. Will the demand for Arzeye Pharma's treatment become more elastic, become less elastic, or not change? Explain.

Point评分要点与解析（本讲义按官方题目自行整理，非官方评分标准）
(a) 1 pt向下倾斜的需求 D，MR 在 D 下方。
(a) 1 ptMC 曲线，在 MR = MC 处标 Q*。
(a) 1 pt从 Q* 往上到需求标 P*。
(a) 1 ptATC 在 Q* 处低于 P*（正的经济利润）。
(a) 1 pt完整涂出消费者剩余：P* 以上、需求以下、从 0 到 Q* 的三角形。
(b)(i) 1 ptQR 在 MR = 0（MR 与横轴的交点）处，位于 Q* 右边。
(b)(ii) 1 ptUnit elastic：TR 最大处 MR = 0，需求单位弹性。
(c)(i) 1 ptP2 在需求与 MC 的交点：完全价格歧视企业会一直卖到顾客愿付的价格等于 MC 为止。
(c)(ii) 1 pt消费者剩余减少到 0（被消除）：每位顾客都被收取自己愿付的最高价，剩余全部变成企业的利润。
(d) 1 ptMore elastic：专利到期后其他企业可以生产仿制药，替代品增多，需求更有弹性。
Quantity of eye treatmentsPrice, Cost0CSDMRMCATCP*Q*QRP2
参考图：Q* 在 MR = MC 处，P* 从需求读，ATC 在 Q* 处低于 P*；蓝色三角形是消费者剩余。QR 在 MR = 0 处（TR 最大）；完全价格歧视时一直卖到 D = MC，最低价格 P2 就在这一点。
FRQ 答题提醒
  • 垄断图的四条线：D、MR（在 D 下方，同一纵截距）、MC、ATC（MC 穿过 ATC 最低点）；价格从需求读。
  • 利润矩形：PM 与 ATC 之间、宽为 QM；DWL 三角形：QM 到 D = MC 交点之间、D 与 MC 之间。
  • “allocatively efficient / socially optimal” → P = MC；“zero economic profit / fair return” → P = ATC。
  • 收益最大 → MR = 0、单位弹性；完全价格歧视 → 卖到 D = MC，消费者剩余为 0，没有 DWL。
接下来 · 下册 介于完全竞争和垄断之间的两种市场：垄断竞争（短期像垄断、长期像完全竞争，但有产能过剩）和寡头（相互依存、博弈论、纳什均衡）。
→ 打开 Unit 4（下）Monopolistic Competition and Oligopoly（与本文件放在同一文件夹）

JL · AP Microeconomics · Unit 4（上）Monopoly and Price Discrimination · 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