JL · AP MICROECONOMICS

Unit 3（下）· Profit and Perfect Competition

利润与完全竞争 — 利润类型、利润最大化、停产与进出、完全竞争 · 对应 CED 3.4–3.7 · Unit 3 选择题占比约 22–25%（上下册合计）
上下册分工 上册：企业内部——投入多少、产出多少、花多少钱（3.1–3.3）。→ 打开 Unit 3（上）Production and Costs
下册（本册）：企业怎样用这些成本做决定——利润类型、利润最大化、停产与进出、完全竞争（3.4–3.7）。
上册回顾：本册要用到的成本知识
  • TC = FC + VC；ATC = AFC + AVC；MC = ΔTC ÷ Δq。
  • MC 先降后升，从下往上穿过 AVC 和 ATC 的最低点。
  • 固定成本变化只移动 ATC；每单位成本变化移动 MC、AVC、ATC。
  • 本册的企业图都用同一套成本曲线：FC = $64，min AVC = $6（q = 6），min ATC = $15（q = 8）。表格题用上册 3.2 的成本表：FC = $100，TC 依次为 100、150、180、200、230、280、360。
Quantity (q)Cost per unit ($)0MCATCAVCAFCmin AVCmin ATCmin MC$66$158
上册的成本曲线（本册所有企业图都用这一套）：MC 先降后升；MC 从下方穿过 AVC 和 ATC 的最低点；AFC 一直下降；ATC 与 AVC 之间的竖直距离就是 AFC，越来越窄。
本册学习目标 LEARNING OBJECTIVES
  • 区分会计利润、经济利润和正常利润 · Distinguish accounting, economic, and normal profit
  • 用总量法和 MR = MC 找出利润最大的产量 · Find the profit-maximizing output
  • 从企业图上读出利润或亏损，算出 TR、TC、TFC · Read profit and loss from a graph
  • 用 P 与 AVC、ATC 的关系判断短期停产和长期进出 · Apply the shutdown rule and explain entry and exit
  • 说出完全竞争的特征，画出并排的市场图和企业图 · Draw side-by-side graphs for a perfectly competitive market and firm
  • 分析需求变化后的短期和长期调整 · Explain short-run and long-run adjustment
  • 解释长期均衡的配置效率和生产效率，区分三种长期行业供给 · Explain efficiency and long-run industry supply
Term 术语Meaning 含义中文
Explicit / Implicit CostActual payments / opportunity cost of the owner's own resources显性 / 隐性成本
Accounting / Economic ProfitTR − explicit costs / TR − explicit − implicit costs会计 / 经济利润
Normal ProfitEconomic profit = 0; owners earn what they could earn elsewhere正常利润
Total / Marginal / Average RevenueTR = P × q; MR = ΔTR/Δq; AR = TR/q总 / 边际 / 平均收益
Profit-Maximizing RuleProduce where MR = MC利润最大化法则
Break-even PricePrice = minimum ATC; economic profit = 0收支相抵价格
Shutdown PricePrice = minimum AVC; below it, produce 0 in the short run停产价格
Short-Run Supply Curve (firm)The MC curve above minimum AVC企业短期供给曲线
Price TakerA firm that cannot affect the market price价格接受者
Entry / ExitFirms join / leave an industry in the long run进入 / 退出
Long-Run EquilibriumP = MR = MC = min ATC; economic profit = 0长期均衡
Allocative / Productive EfficiencyP = MC / P = min ATC配置 / 生产效率
Constant / Increasing / Decreasing-Cost IndustryLong-run industry supply is horizontal / upward / downward sloping成本不变 / 递增 / 递减行业
考试怎么考 选择题：经济利润与会计利润计算、MR = MC 找产量、从图上读利润、停产点与收支相抵点、企业短期供给曲线、需求变化后的短期与长期调整、长期均衡的效率。FRQ：几乎每年都有完全竞争的并排市场图和企业图（标价格、产量、涂利润或亏损），常和固定成本、补贴、长期调整一起考。

3.4Types of Profit 利润的类型

Unit 1.5 已经接触过显性成本和隐性成本。这一节把它们用在企业身上：企业是否进入或退出一个行业，看的是经济利润，而不是会计账本上的利润（CED CBA-2.C.1）。

定义 DEFINITION Accounting profit = Total revenue − Explicit costs Economic profit = Total revenue − Explicit costs − Implicit costs 隐性成本包括：老板放弃的工资（企业家的时间）、自有资金本可赚到的利息（financial capital 的机会成本）、承担风险应得的回报（CED CBA-2.C.2）。
Normal Profit 正常利润 当经济利润 = 0 时，会计利润正好等于隐性成本——所有资源（包括老板自己的时间和钱）都拿到了它们在次优用途上能拿到的回报。这叫 earning a normal profit。
所以“经济利润为 0”不是不赚钱：老板赚到的钱和去别处工作、把钱存银行一样多，没有动力离开，也没有额外的吸引力让别人进来。
两种利润的大小关系 因为隐性成本 ≥ 0，所以会计利润 ≥ 经济利润。会计利润为正，经济利润可能为正、为零，也可能为负；经济利润为正，会计利润一定为正。
WORKED EXAMPLE 1开健身房值不值？ 原创

Lin quits a job paying $90,000 a year and uses $200,000 of her own savings (which could earn 5% interest) to open a gym. In the first year the gym earns $300,000 in revenue. She pays $60,000 rent, $80,000 in wages, $30,000 to lease equipment, and $10,000 for utilities. Calculate (a) explicit costs, (b) implicit costs, (c) accounting profit, (d) economic profit. (e) Should Lin keep the gym open in the long run?

(a) 显性成本 = 60 + 80 + 30 + 10 = $180,000。
(b) 隐性成本 = 放弃的工资 90,000 + 放弃的利息 200,000 × 5% = 10,000 → $100,000。
(c) 会计利润 = 300,000 − 180,000 = $120,000。
(d) 经济利润 = 120,000 − 100,000 = $20,000。
(e) 经济利润 > 0，比她的次优选择（上班 + 存钱）多赚 $20,000，应该继续经营。如果其他人看到健身行业有正的经济利润，也会进入这个行业（见 3.6、3.7）。
总收入Revenue 300会计师看显性成本 180会计利润 120经济学家看显性 180隐性 100← 经济利润 20
单位：千美元。隐性成本 100 = 放弃的工资 90 + 自有资金本可赚的利息 10。
WORKED EXAMPLE 2餐车老板的“正常利润”是多少？ 原创

Marco left a job paying $50,000 a year and invested $100,000 of his savings (which could earn 5% interest) in a food truck. (a) How much accounting profit must the truck earn for Marco to earn a normal profit? (b) This year accounting profit is $60,000. What is his economic profit? (c) If accounting profit falls to $55,000, should Marco quit?

(a) 隐性成本 = 50,000 + 100,000 × 5% = $55,000。会计利润 = $55,000 时经济利润为 0，这 $55,000 就是他的正常利润。
(b) 经济利润 = 60,000 − 55,000 = $5,000。
(c) 经济利润 = 0，他的收入和去上班、把钱存银行一样多，没有理由离开——不必退出。只有经济利润为负时，他才应该转行。
WORKED EXAMPLE 3会计上赚了 $40,000，为什么还该关店？ 原创

Amy turned down a job offer paying $70,000 a year to run her own bubble tea shop. She rents the shop and all the equipment, and she used none of her own savings. This year the shop's revenue is $260,000 and its explicit costs are $220,000. (a) Calculate accounting profit. (b) Calculate economic profit. (c) A friend says, "You made a $40,000 profit, keep going!" Is the friend right? (d) If this continues, what should Amy do in the long run?

(a) 会计利润 = 260,000 − 220,000 = $40,000。
(b) 隐性成本 = 放弃的工资 $70,000（没有用自己的钱，所以没有利息这一项）。经济利润 = 40,000 − 70,000 = −$30,000。
(c) 只看会计账本是对的，但经济学看机会成本：Amy 去上班能多拿 $30,000，开店让她“少赚了” $30,000。
(d) 经济利润 < 0，长期应该退出（关店去上班）。所以“会计利润 > 0”不能保证企业留在行业里（真题 2022 国际卷 Q1 就考这一点）。
Lin 的健身房会计利润 120隐性成本 100经济利润 = 120 − 100 = +20 → 比次优选择多赚，别人也想进入这个行业Marco 的餐车会计利润 55隐性成本 55经济利润 = 55 − 55 = 0 → 正常利润，留下来经营，没人急着进出Amy 的奶茶店会计利润 40隐性成本 70经济利润 = 40 − 70 = −30 → 会计上赚钱，经济上亏钱，长期应该退出
单位：千美元。绿色是会计利润，黄色是隐性成本（老板放弃的工资、自有资金的利息）。绿条比黄条长，经济利润为正；一样长，经济利润为 0；绿条更短，经济利润为负。
LAB 1显性还是隐性？ · Explicit or Implicit?

Lin 辞掉年薪 $90,000 的工作，用自己的 $200,000 积蓄开了一家健身房。判断下面每一项是显性成本还是隐性成本。（得分：0 / ）

练习 Practice 3.4

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。

3.4-12016 AP Micro 练习卷 Q22
Shelby is an entrepreneur who has decided to open a small advertising firm. She rents office space at a cost of $25,000 per year, she has employed an assistant at a salary of $30,000 per year, and she incurs annual utility and office supply expenses of $20,000. Her best alternative is to work elsewhere and to earn a salary of $50,000 per year. How much annual revenue must her firm receive so that Shelby earns zero economic profit?
  • $50,000
  • $75,000
  • $100,000
  • $125,000
  • $150,000
零经济利润要求收入覆盖显性成本 25,000 + 30,000 + 20,000 = 75,000，加上隐性成本（放弃的工资）50,000，合计 $125,000。
3.4-22016 AP Micro 练习卷 Q23
A firm produces 400 books and sells each book for $15. If the explicit cost of producing the books is $4,500 and the implicit cost is $1,000, the firm’s economic profit is
  • $0
  • $500
  • $1,000
  • $1,500
  • $5,000
TR = 400 × 15 = 6,000；经济利润 = 6,000 − 4,500 − 1,000 = $500。
3.4-32018 AP Micro 练习卷 Q9
Bruce is a talented writer and graphic artist who enjoys both types of work equally. Instead of earning $45,000 as a writer, Bruce now earns $25,000 in accounting profits as a graphic artist using the same computer equipment he would have used as a writer. What is Bruce’s economic profit from choosing to work as a graphic artist?
  • -$45,000
  • -$20,000
  • $20,000
  • $45,000
  • $70,000
经济利润 = 会计利润 − 放弃的次优收入 = 25,000 − 45,000 = −$20,000。
3.4-42019 AP Micro 练习卷 Q54
Jamal quits a job that was paying him $30,000 per year and decides to start his own business. He runs his business out of his house in a room he had been renting to his colleague for $12,000 a year. Jamal withdraws the $20,000 in his savings account that had been earning him a 10 percent annual interest to purchase computers and related accessories and equipment for the business. During the first year of operation, Jamal’s business incurred $30,000 in explicit costs and generated $60,000 in total sales. Jamal’s economic profit is
  • $30,000
  • $17,000
  • $0
  • – $2,000
  • – $14,000
会计利润 = 60,000 − 30,000 = 30,000。隐性成本 = 放弃的工资 30,000 + 放弃的房租 12,000 + 放弃的利息 20,000 × 10% = 2,000，共 44,000。经济利润 = 30,000 − 44,000 = −$14,000。
3.4-52021 AP Micro 国际卷 Q1
Raheem is currently working as a financial analyst earning $75,000 a year and is considering quitting his current job to start an art gallery. The estimated annual revenue from the art gallery is $175,000. The annual cost of labor, advertising, and acquiring the art inventory is $125,000. What are Raheem’s accounting and economic profits if he opens the art gallery?
  • Accounting profit is −$25,000, and economic profit is $50,000.
  • Accounting profit is $100,000, and economic profit is $50,000.
  • Accounting profit is $50,000, and economic profit is −$25,000.
  • Accounting profit is $100,000, and economic profit is −$25,000.
  • Accounting profit is $50,000, and economic profit is $100,000.
会计利润 = 175,000 − 125,000 = 50,000；经济利润 = 50,000 − 75,000（放弃的工资）= −25,000。
3.4-62023 AP Micro 国际卷 Q15
Dylan recently quit his job as a journalist for a local newspaper where he was making $80,000 per year. He opened an ice-cream shop where he earns a yearly total revenue of $250,000. He spends $150,000 each year on rent, resources, and workers he hires. Which of the following equals Dylan’s accounting profit and economic profit from the ice-cream shop?
  • Accounting Profit: $250,000；Economic Profit: $20,000
  • Accounting Profit: $230,000；Economic Profit: $100,000
  • Accounting Profit: $150,000；Economic Profit: $70,000
  • Accounting Profit: $100,000；Economic Profit: $80,000
  • Accounting Profit: $100,000；Economic Profit: $20,000
会计利润 = 250,000 − 150,000 = 100,000；经济利润 = 100,000 − 80,000 = 20,000。
3.4-72025 AP Micro 国际卷 Q49
Nikita was working at a law firm earning an annual salary of $80000, but she decided to start her own law practice. She used a building she owns and had been renting to a tenant for $15000 a year. Her law practice earned an accounting profit of $90000 in its first year. Based on the preceding information, what was Nikita's economic profit?
  • −$80,000
  • −$15,000
  • −$5,000
  • $10,000
  • $75,000
经济利润 = 会计利润 − 隐性成本 = 90,000 − 80,000（放弃的工资）− 15,000（放弃的租金）= −$5,000。

3.5Profit Maximization 利润最大化

经济学假设企业的目标是经济利润最大（CED CBA-2.D.1）。和 Unit 1.6 的“MB = MC”是同一个思路，只是名字换成了收入和成本。

收入 REVENUE Total revenue TR = P × q；Marginal revenue MR = ΔTR ÷ Δq（多卖一单位增加的收入）；Average revenue AR = TR ÷ q = P。
完全竞争企业是 price taker，无论卖多少价格都不变，所以 MR = AR = P。（Unit 4 的垄断企业要降价才能多卖，MR < P。）
利润最大化法则 PROFIT-MAXIMIZING RULE Produce where MR = MC 对所有市场结构的企业都成立。表格题中如果没有恰好相等的产量：只要 MR ≥ MC 就继续生产，做到最后一个 MR ≥ MC 的单位为止。
WORKED EXAMPLE 4价格 $60 时生产多少？ 原创

A perfectly competitive firm has the cost schedule from 3.2 (FC = $100) and sells at a market price of $60. (a) Use the total approach to find the profit-maximizing output. (b) Confirm using marginal analysis. (c) What is the maximum profit?

qTRTCProfit = TR − TCMRMC
00100-100——
160150-906050
2120180-606030
3180200-206020
4240230106030
5300280206050
636036006080
(a) 利润一列最大的是 q = 5（$20）。
(b) 第 1–5 个单位都满足 MR（60）≥ MC（50, 30, 20, 30, 50）；第 6 个单位 MC = 80 > 60，不该生产。所以 q* = 5。
(c) 利润 = 300 − 280 = $20。也可以用平均值算：(P − ATC) × q = (60 − 56) × 5 = $20。

Total vs. Marginal Approach 总量法与边际法

Dollars ($)TCTR = 21q利润 = TR − TC（绿色竖线，最大 ≈ $51）虚线：与 TR 平行的切线Quantity (q)$0MCMR = P$21MR = MCq* ≈ 8.9MR > MC：多生产MR < MC：少生产
上下两张图横轴对齐，价格 P = $21。上图（总量法）：TR 与 TC 之间的竖直距离就是利润，在 q* ≈ 8.9 处最大——这里 TC 的切线（虚线）与 TR 平行，也就是 TC 的斜率（MC）= TR 的斜率（MR）。下图（边际法）：同一个 q* 正是 MR = MC 的地方。两种方法得到同一个答案。
易错 TRAP
  • 利润最大 ≠ TR 最大，也 ≠ ATC 最低。题目问“profit-maximizing quantity”，只看 MR = MC。
  • 利润最大也可能是“亏损最小”（价格低于 ATC 时），规则不变。
  • MC 曲线和 MR 线可能相交两次：要选 MC 从下往上穿过 MR 的那个点（见下图）。
Quantity (q)$0MCMR = P$10ABA：MC 下降时与 MR 相交 ✗B：MC 上升时相交 ✓
MC 与 MR 可能相交两次。在 A 点之后再多生产，MC 低于 MR，每多一单位都多赚钱，所以 A 不是利润最大点；要选 MC 从下往上穿过 MR 的 B 点。
WORKED EXAMPLE 5为什么不在 ATC 最低处生产？ 原创

Using the cost curves in this unit (minimum ATC = $15 at q = 8), suppose the market price is $21. Compare the firm's profit if it produces at minimum ATC (q = 8) with its profit at the quantity where MR = MC.

q = 8（ATC 最低）：利润 = (21 − 15) × 8 = $48。
q* ≈ 8.9（MR = MC）：ATC ≈ $15.29，利润 ≈ (21 − 15.29) × 8.9 ≈ $51。
虽然多生产让每单位的平均成本略有上升，但第 8 到第 8.9 单位的 MC 仍低于价格 $21，每一单位都在多赚钱。企业关心的是总利润，不是每单位成本最低。只有在长期均衡（P = min ATC）时，两个产量才正好重合。
LAB 2找出利润最大的产量 · Find the Best Output

用 3.2 的成本表（FC = $100）。改变市场价格，点一点表格中你认为利润最大的那一行，再对照 MR = MC 规则。

市场价格 P = MR = $60
点击表格中的一行作答。

练习 Practice 3.5

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。

3.5-12000 AP Micro Q38
A perfectly competitive firm, earning economic profits, produces and sells 100 units of output at a price of $20 per unit. If its marginal cost of increasing output to a rate of 101 units is $18, which of the following statements is correct?
  • The total revenue from selling 101 units is the same as the total revenue from selling 100 units.
  • The total profit from selling 101 units is $2 greater than the total profit from selling 100 units.
  • The total cost of producing 101 units is $2 greater than the total cost of producing 100 units.
  • To sell 101 units, the firm must reduce its price below $20.
  • To sell 101 units, the firm must raise its price above $20.
第 101 单位：MR = P = $20，MC = $18，利润增加 20 − 18 = $2。完全竞争企业不用降价就能多卖。
3.5-22010 AP Micro Q10
At the current output level, a firm finds that it has the potential to increase its profit by expanding output. If P = price, MR = marginal revenue, and MC = marginal cost, which of the following must hold at the current output for this firm?
  • P = MR < MC
  • P = MR = MC
  • MR = MC
  • MR > MC
  • MR < MC
还能通过增产提高利润，说明下一单位的 MR > MC。
3.5-32012 AP Micro Q53
If total revenue is increasing as output increases, marginal revenue is always
  • equal to average revenue
  • less than average revenue
  • increasing
  • decreasing
  • greater than zero
TR 随产量增加 → 每多卖一单位收入都增加 → MR > 0。
3.5-42013 AP Micro Q16
If a perfectly competitive firm is producing where marginal cost is rising and greater than marginal revenue, to maximize profits it should
  • increase the level of production
  • decrease the level of production
  • maintain current level of production
  • increase price
  • decrease price
MC > MR：最后几单位的成本高于收入，减产能提高利润。完全竞争企业不能自己定价，(D)(E) 错。
3.5-52014 AP Micro 练习卷 Q25
The most profitable level of output for any firm operating in the short run is the level of output at which
  • marginal revenue exceeds marginal cost by the highest amount
  • marginal revenue equals marginal cost
  • price exceeds average cost by the highest amount
  • price equals average cost
  • price equals marginal cost
任何企业利润最大的产量都在 MR = MC 处。(E) P = MC 只对价格接受者成立（因为 P = MR）。
3.5-62017 AP Micro 练习卷 Q4
At a perfectly competitive firm’s current output level, average total cost is $15, average variable cost is $10, and marginal cost is $8 and increasing. If the product price is $15, what should this firm do to maximize profits?
  • Increase the quantity of output produced.
  • Increase the product price.
  • Decrease the product price to increase sales.
  • Shut down immediately.
  • Continue to produce at its current output level.
P = MR = $15 > MC = $8 → 多生产一单位还能多赚 → 增产，直到 MC 上升到 $15。
3.5-72019 AP Micro 练习卷 Q41
At the current quantity that a firm is selling, the firm has marginal revenue of $750 and marginal cost of $800. Which of the following is true?
  • The firm is maximizing profit.
  • The firm’s profits would increase if the firm increased the quantity sold.
  • The firm’s profits would increase if the firm decreased the quantity sold.
  • The firm earns negative economic profit.
  • The firm earns zero accounting profit.
MR（750）< MC（800）：最后一单位让利润减少 50 → 减少销量能提高利润。
3.5-82019 AP Micro 练习卷 Q56
A perfectly competitive firm is producing 10 units of output and sells the product for $5 per unit. At this level of output the average total cost is $4, the average variable cost is $3 and the marginal cost is $7. What should this firm do to maximize short-run profits?
  • Increase output until price equals average total cost.
  • Increase output until price equals marginal cost.
  • Leave output unchanged because price is greater than average total cost.
  • Decrease output until price is equal to marginal cost.
  • Decrease output until price is equal to average total cost.
MC（$7）> P（$5）：最后几单位亏钱，应减产到 P = MC。虽然现在有利润（P > ATC），但不是最大利润。
3.5-92025 AP Micro 国际卷 Q26
Quantity (bushels)Total cost ($)
010
112
216
322
432
550
669
The table shows the total cost schedule for a soybean farm.

The farm grows and sells soybeans at a price of $15 per bushel in a perfectly competitive market. What is the farm’s total economic profit at the profit-maximizing quantity of output?
  • $10
  • $21
  • $23
  • $25
  • $28
FC = 10，MC 依次为 2、4、6、10、18、19。P = $15，生产到最后一个 MC ≤ 15 的单位 → q = 4。利润 = 15 × 4 − 32 = $28。
3.5-102012 AP Micro Q52
QUANTITYCOST/REVENUE0Total RevenueTotal CostQ₁Q₂Q₃Q₄
按原题重绘
The graph above shows the total revenue and total cost curves for a firm in which type of market structure and what is the profit-maximizing quantity?
  • Market Structure: Monopoly；Quantity: Q2
  • Market Structure: Monopoly；Quantity: Q3
  • Market Structure: Perfect Competition；Quantity: Q1
  • Market Structure: Perfect Competition；Quantity: Q3
  • Market Structure: Perfect Competition；Quantity: Q4
TR 是从原点出发的直线 → 价格不随产量变 → 完全竞争。TR 在 TC 上方、竖直距离最大处是 Q₃。Q₂ 和 Q₄ 是收支相抵点（利润 = 0）。
3.5-112023 AP Micro 国际卷 Q37
QuantityPrice0MCATCAVCMRQ*
按原题重绘
The graph provided shows the marginal revenue (MR), average variable cost (AVC), average total cost (ATC), and marginal cost (MC) curves for a perfectly competitive firm. Which of the following is true if the firm is currently producing Q* units of output?
  • The firm can increase its economic profit by producing fewer units of output.
  • The firm can increase its economic profit by increasing the price of its product.
  • The firm is maximizing its economic profit at the current level of output.
  • The firm can increase total revenue by producing fewer units of output.
  • The firm can increase total revenue by decreasing the price of its product.
在 Q* 处 MC（= min ATC）高于 MR → 最后几单位多花的成本比多得的收入多 → 减少产量能增加利润（或减少亏损）。完全竞争企业不能自己提价（B、E 错）；少生产 TR 会下降（D 错）。
3.5-122025 AP Micro 国际卷 Q20
QuantityPrice, Cost ($)0Marginal CostAverage Total CostAverage Variable Costdemand = Marginal Revenue2616565220284029
按原题重绘
The graph shows the cost and revenue curves for the firm LuxEcon.

LuxEcon operates in a perfectly competitive market and produces the profit-maximizing quantity of output. Which of the following correctly identifies LuxEcon’s profit-maximizing quantity and economic profit?
  • LuxEcon’s profit-maximizing quantity is 16, and LuxEcon earns positive economic profit of $256.
  • LuxEcon’s profit-maximizing quantity is 16, and LuxEcon earns negative economic profit of $256.
  • LuxEcon’s profit-maximizing quantity is 20, and LuxEcon earns positive economic profit of $240.
  • LuxEcon’s profit-maximizing quantity is 20, and LuxEcon earns negative economic profit of $240.
  • LuxEcon’s profit-maximizing quantity is 20, and LuxEcon earns negative economic profit of $480.
MR = MC 在 q = 20；该处 ATC = $52 > P = $40 → 经济利润 = (40 − 52) × 20 = −$240。P = 40 > AVC = 28，短期继续生产。q = 16 是 AVC 最低点，不是利润最大的产量。

3.6Short-Run Shutdown & Long-Run Entry/Exit 短期停产与长期进出

Profit and Loss on the Graph 在图上读利润和亏损

公式 FORMULA Profit = (P − ATC) × q* 先在 MR = MC 处找到 q*，再往上找到 q* 对应的 ATC，P 与 ATC 之间的矩形就是利润（P > ATC）或亏损（P < ATC）。
Quantity (q)$0④ ProfitMCATCAVCd = MR = P$21$15.3q* ≈ 8.9① MR = MC③ ATC② 往下找 q*
读利润四步：① 在 MR = MC 处找交点；② 往下读出 q* ≈ 8.9；③ 从 q* 往上找到 ATC ≈ $15.3；④ P 与 ATC 之间、宽为 q* 的矩形就是利润 = (21 − 15.3) × 8.9 ≈ $51。ATC 在 P 下方是利润（绿色），在 P 上方就是亏损（红色）。
Quantity (q)$0Loss ≈ 38ATC=15.3MCATCAVCD = MR = P$10q*≈7.1
P = $10：低于 ATC，亏损 ≈ (15.3 − 10) × 7.1 ≈ $38（红色矩形）。但 P 仍高于 AVC，继续生产能覆盖全部可变成本和一部分固定成本——亏 $38 好过停产亏掉全部固定成本 $64。
WORKED EXAMPLE 6从图上读出利润、TR、TC 和 TFC 原创 · AP 读图考法

The graph shows the cost and revenue curves for LuxPet, a profit-maximizing firm in a perfectly competitive market. Identify or calculate (a) the profit-maximizing quantity, (b) total revenue, (c) total cost, (d) economic profit, (e) total fixed cost, and (f) the lowest price at which LuxPet would keep producing in the short run.

QuantityPrice, Cost ($)0MCATCAVCd = MR$3820$30$18$1514
LuxPet 的成本与收益曲线（完全竞争企业）
(a) MR = MC 在 q = 20（d = MR 线与 MC 的交点）。注意 14 是 AVC 最低点，不是答案。
(b) TR = P × q = 38 × 20 = $760。
(c) TC = ATC × q = 30 × 20 = $600。
(d) 利润 = 760 − 600 = $160，或 (P − ATC) × q = (38 − 30) × 20。
(e) TFC = (ATC − AVC) × q = (30 − 18) × 20 = $240（ATC 与 AVC 的竖直距离是 AFC）。
(f) 停产价格 = min AVC = $15（q = 14 处，MC 穿过 AVC 的最低点）。
QuantityPrice, Cost ($)0利润 $160MCATCAVCd = MR$3820$30$18$1514
q* = 20 处 ATC = $30 < P = $38，绿色矩形就是经济利润 $160。

The Shutdown Rule 短期停产规则（CED PRD-2.A.1）

亏损了是不是马上关门？不一定。短期内固定成本（房租、贷款）关门也照样要付，所以要比较的是“营业”和“关门”哪个亏得少：

短期决策
  • P ≥ AVC（TR ≥ TVC）：继续生产。收入覆盖了全部可变成本，还能贴补一部分固定成本，亏损 ≤ FC。
  • P < AVC（TR < TVC）：停产，产量为 0，亏损 = FC。营业的话连原料、工资都赚不回来，亏得更多。
  • 收支相抵价格（break-even price）= min ATC；停产价格（shutdown price）= min AVC。
P = $10营业（q ≈ 7.1）亏 $38停产 亏 FC = $64营业亏得少 → 继续生产（TR ≈ $71 > TVC ≈ $44）P = $5营业（q ≈ 5.6）亏 $70停产 亏 FC = $64停产亏得少 → 停产（TR ≈ $28 < TVC ≈ $34）
停产不等于不亏钱：短期内固定成本 $64 照付。所以比较的是“营业亏多少”和“停产亏 $64”，等价于比较 TR 与 TVC（或 P 与 AVC）。
Quantity (q)$0MCATCAVC收支相抵价格 = min ATC = $15$15停产价格 = min AVC = $6$686
同一套成本曲线上只有两个关键价格：break-even price 在 ATC 最低点（$15），shutdown price 在 AVC 最低点（$6）。价格落在哪个区间，企业怎么做，见下表。
市场价格短期产量（MR = MC）经济利润短期决策长期
P > $15q > 8> 0生产新企业进入，P 回落到 $15
P = $15
收支相抵点		q = 8= 0（正常利润）生产不进不退
$6 < P < $156 < q < 8< 0，但亏损 < FC继续生产企业退出，P 回升到 $15
P = $6
停产点		q = 6 或 0= −FC = −$64生产与停产一样退出
P < $60= −FC = −$64停产退出
Quantity (q)$0ATCAVCS = MC（AVC 以上的部分）shutdown point$66
完全竞争企业的短期供给曲线 = MC 曲线在 AVC 最低点以上的部分；价格低于 $6 时产量为 0（纵轴上那一段）。把所有企业的供给曲线横向相加，就是 Unit 2 的市场供给曲线。
WORKED EXAMPLE 7价格下跌：$50、$35、$30 时怎么办？ 原创

Using the cost table from 3.2 (FC = $100, minimum AVC = $32.50 at q = 4), find the firm's output and profit (or loss) when the market price is (a) $50, (b) $35, (c) $30. In each case, should the firm produce or shut down in the short run?

(a) P = $50：MR ≥ MC 到第 5 单位（MC₅ = 50）→ q = 5，TR = 250，TC = 280，亏 $30。P > AVC → 继续生产（停产要亏 $100）。
(b) P = $35：MC ≤ 35 到第 4 单位（MC₄ = 30，MC₅ = 50）→ q = 4，TR = 140，TC = 230，亏 $90。TR 140 > VC 130，P = 35 > AVC = 32.5 → 继续生产（亏 90 < 100）。
(c) P = $30：如果生产 q = 4，TR = 120 < VC = 130，亏损 = 230 − 120 = $110 > 100 → 停产，只亏固定成本 $100。
结论：这家企业的停产价格是 min AVC = $32.5，收支相抵价格是 min ATC = $56。
固定成本变化不影响停产决策 房租上涨只会改变 ATC（亏得更多），但不改变 MC 和 AVC，所以短期内“生产多少、要不要停产”都不变。这和上册 3.2 的 lump-sum tax 是同一个道理。
WORKED EXAMPLE 8淡季的电影院：开门还是关门？ 原创

A cinema pays $10,000 a week in rent and loan payments no matter what. If it stays open, it pays $12,000 a week for staff, electricity, and film rental. (a) Weekly ticket revenue is $15,000. Should it stay open in the short run? (b) What if revenue falls to $10,000? (c) What will happen in the long run if revenue stays at $15,000?

(a) 营业：亏 10,000 + 12,000 − 15,000 = $7,000；关门：亏固定成本 $10,000。TR（15,000）> TVC（12,000）→ 继续营业。
(b) 营业：亏 22,000 − 10,000 = $12,000 > 10,000 → TR < TVC → 暂时关门。
(c) 每周都在亏损（经济利润 < 0），租约和贷款到期后不再续——长期中所有成本都可变，影院会退出这个行业，除非票房回升。

Long-Run Entry and Exit 长期进入与退出（CED PRD-2.A.2）

🚪
经济利润 > 0 → 行业外的人看到“比别处赚得多” → 进入（没有进入壁垒时）。
🏃
经济利润 < 0 → 长期中所有成本都是可变成本，不再有“非付不可”的固定成本 → 只要 P < ATC 就会退出。
⚖️
经济利润 = 0（正常利润）→ 没人进、没人走 → 长期均衡。
易错 TRAP · 短期停产 vs 长期退出 短期看 P 与 AVC（还有固定成本要付）；长期看 P 与 ATC（没有固定成本了）。所以 $6 < P < $15 时：短期继续生产，长期退出。
LAB 3企业决策实验室 · Produce, Lose, or Shut Down?

成本曲线：min AVC = $6（q = 6），min ATC = $15（q = 8），FC = $64。拖动市场价格，看企业在 MR = MC 处生产多少、赚还是亏、该不该停产。

市场价格 P = $21

练习 Practice 3.6

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。

3.6-12000 AP Micro Q25
In a perfectly competitive industry, the market price of the product is $12. A firm produces at a level of output where average total cost is $16, marginal cost is $16, and average variable cost is $8. To maximize its profit, the firm should
  • decrease its selling price
  • increase its selling price
  • decrease output but keep producing
  • shut down
  • leave both price and output unchanged
MC（16）> P（12）→ 应减产；P（12）> AVC（8）→ 不应停产。
3.6-22005 AP Micro Q40
In the short run, a perfectly competitive firm should shut down whenever
  • minimum average variable cost exceeds price
  • minimum average total cost exceeds price
  • marginal revenue exceeds marginal cost
  • marginal cost exceeds marginal revenue
  • it is losing money
短期停产条件：价格低于最低 AVC。只是亏损（P < ATC）不一定停产。
3.6-32012 AP Micro Q41
In order to minimize short-run losses, a profit-maximizing firm will necessarily shut down production under which of the following conditions?
  • Total revenue is less than total cost.
  • Marginal cost is greater than average total cost.
  • Marginal cost is less than marginal revenue.
  • Average revenue is less than average variable cost.
  • Average revenue is less than average total cost.
AR（= P）< AVC 时，生产连可变成本都赚不回来 → 停产。TR < TC 只是亏损，未必停产。
3.6-42013 AP Micro Q25
Assume that, for a perfectly competitive firm, marginal cost equals average variable cost at $10, marginal cost equals average total cost at $15, and marginal revenue equals marginal cost at $12. On the basis of this information, the firm should
  • close down in the short run
  • operate in the short run, even though it will sustain a loss
  • operate in the short run, because it will make an economic profit of $3 per unit
  • operate in the long run, because it will make an economic profit of $3 per unit
  • operate in the short run, but decrease output to decrease its cost
MR = MC 在 $12，即 P = $12：高于 min AVC（10），低于 min ATC（15）→ 短期继续生产，但有亏损。
3.6-52015 AP Micro 练习卷 Q9
Suppose that price in a perfectly competitive industry decreases and it is now below minimum average total cost but remains above minimum average variable cost. Which of the following will occur in the short run?
  • New firms will enter the industry.
  • Firms will increase output so that marginal revenue equals the new price.
  • Firms will produce the output at which average total cost is at a minimum.
  • Firms will produce the output at which marginal cost equals the new price.
  • Firms will not produce at all, since they will be unable to cover all their costs.
min AVC < P < min ATC：短期继续生产，在新的 P = MC 处生产（亏损最小）。长期才会有企业退出。
3.6-62015 AP Micro 练习卷 Q52
In the short run, if a firm produces the level of output at which marginal revenue is equal to marginal cost but price is less than average total cost, the firm will
  • always shut down production
  • expand output to lower its average fixed cost
  • continue to operate if price is greater than its average variable cost
  • decrease output until price equals its average total cost
  • increase output to increase revenue
P < ATC 时有亏损；只要 P > AVC，继续生产能贴补一部分固定成本，比停产亏得少。
3.6-72016 AP Micro 练习卷 Q10
Assume that a profit-maximizing, perfectly competitive firm has economic losses in the short run. If the firm continues to produce and sell its goods, then which of the following must be true?
  • The firm is covering all of its fixed and variable costs of production.
  • The firm is covering all of its fixed costs but not all of its variable costs of production.
  • The firm is covering all of its variable costs but not all of its fixed costs of production.
  • The firm is covering all of its implicit costs but not all of its explicit costs.
  • The firm must have raised the price of its goods in order to minimize its losses.
亏损但继续生产 → P ≥ AVC：收入覆盖了全部可变成本，只覆盖了一部分固定成本。
3.6-82016 AP Micro 练习卷 Q21
Daily outputTotal variable cost
0$0
120
230
348
472
5100
Based on the cost and output data in the table above, a perfectly competitive firm will shut down if price falls below
  • $30
  • $20
  • $18
  • $16
  • $15
AVC = TVC ÷ q：20、15、16、18、20。最低 AVC = $15（q = 2），价格低于 $15 就停产。
3.6-92018 AP Micro 练习卷 Q12
A perfectly competitive firm’s short-run supply curve is the portion of the marginal cost curve that is
  • above the average total cost curve
  • above the average variable cost curve
  • above the minimum average fixed cost curve
  • above the demand curve
  • between the average fixed cost curve and the average variable cost curve
完全竞争企业的短期供给曲线 = MC 曲线在 AVC（最低点）以上的部分。
3.6-102019 AP Micro 练习卷 Q24
Assume a perfectly competitive firm is currently producing 100 units of output. Its marginal cost is $6 and rising at that output quantity. Its average variable cost is $7 and its average fixed cost is $3. If the product’s price is $6, which of the following will the firm do in the short run to maximize its profit?
  • Shut down
  • Produce, but less than 100 units of output
  • Produce more than 100 units of output
  • Continue to produce at exactly 100 units of output
  • Increase its price above $6
P = $6 < AVC = $7 → 短期停产。（这时 MC = P，但价格连 AVC 都覆盖不了。）
3.6-112013 AP Micro Q23
Question 23 is based on the graph below, which shows the cost curves for a perfectly competitive firm.
QUANTITYPRICE0Marginal CostAverage Total CostAverage Variable CostP₁P₂P₃RQ₁SQ₂TQ₃UQ₄V
按原题重绘
Which of the following segments of the marginal cost curve lies entirely on the firm’s short-run supply curve?
  • TUV
  • STU
  • RSTU
  • RSTUV
  • RS
企业短期供给曲线 = MC 在 AVC 最低点（T）以上的部分 → TUV。R、S 在 AVC 最低点以下，价格那么低时企业停产。
3.6-122014 AP Micro 练习卷 Q35
Questions 35-36 refer to the graph below for a representative firm in a perfectly competitive, constant-cost industry, which shows the firm’s marginal cost (MC), average total cost (ATC), and average variable cost (AVC).
QUANTITYCOSTS0MCATCAVCP₁Q₁P₂Q₂P₃Q₃P₄
按原题重绘
In the short run, the firm will realize an economic loss but will continue to produce if the price is
  • below P1
  • between P1 and P2
  • below P2
  • between P2 and P3
  • between P3 and P4
P₂ = min AVC（停产点），P₃ = min ATC（收支相抵点）。价格在 P₂ 与 P₃ 之间：P < ATC 所以亏损，但 P > AVC 所以继续生产。
3.6-132021 AP Micro 国际卷 Q48
QuantityCost, Price0MCATCAVCMR = dPQ₁Q₂Q₃Q₄
按原题重绘
The graph provided shows the demand (d), marginal revenue (MR), marginal cost (MC), and average total cost (ATC) curves for a perfectly competitive firm. In order to maximize short-run profits, this perfectly competitive firm should produce at which of the following quantities of production?
  • Zero
  • Q1
  • Q2
  • Q3
  • Q4
P 低于 AVC 的最低点（Q₃ 处）→ 无论生产多少都收不回可变成本 → 停产，产量为 0。Q₂ 虽然 MR = MC，但那里 P < AVC。
3.6-142012 AP Micro Q38
QUANTITYPRICE0Marginal CostAverage Total CostAverage Variable CostP₁
按原题重绘
The graph above shows the short-run cost curves of a firm in a perfectly competitive market. Which of the following are true at the firm’s profit-maximizing output level?
I. Price exceeds average total cost.
II. Economic profits are zero.
III. Marginal cost equals average total cost.
IV. New firms are likely to enter the market in the long run.
  • I and II only
  • I and III only
  • I and IV only
  • II, III, and IV only
  • I, II, III, and IV
P₁ = MC 的产量在 ATC 最低点右边，此处 ATC < P₁ → I 对，有经济利润（II 错）；此处 MC > ATC（III 错）；有经济利润 → 新企业进入（IV 对）。

3.7Perfect Competition 完全竞争

四个特征 CHARACTERISTICS（CED PRD-3.A.1）
  • Many small firms 大量小企业：每家的产量占市场的比例微不足道。
  • Identical (standardized) products 产品完全相同：买家不在乎买谁家的。
  • Free entry and exit 没有进入和退出壁垒。
  • Perfect information 买卖双方都知道市场价格。
结果：每家企业都是 price taker，没有定价能力（no market power）。例：小麦、玉米、鸡蛋批发、农贸市场里卖青椒的摊位。

Market vs. Firm 市场图与企业图

完全竞争要画两张图：市场的供求决定价格，单个企业把这个价格当作给定的，面对一条水平的需求曲线（CED PRD-3.A.3–4）。几乎每年的 FRQ 都要求画这两张并排的图，而且两张图的价格要对齐。

为什么企业的需求曲线是水平的？ 一个小麦农民如果比市场价贵一分钱，买家立刻去买别人家一模一样的小麦，他一斤也卖不出去；如果比市场价便宜，他自己就少赚了——反正按市场价想卖多少都卖得掉。所以企业面对的需求完全有弹性：D = MR = AR = P。注意市场需求曲线仍然向下倾斜。
WORKED EXAMPLE 9FRQ 必考：完全竞争 side-by-side 图分四步画 原创 · AP FRQ 画法

The market for eggs is perfectly competitive and in long-run equilibrium. Draw correctly labeled side-by-side graphs for the egg market and for a typical egg farm, and show each of the following: (i) the market equilibrium price and quantity, labeled PM and QM; (ii) the farm's price and profit-maximizing quantity, labeled PF and qF; (iii) the farm's average total cost curve consistent with long-run equilibrium, labeled ATC.

Market 市场Firm 企业Quantity (Q)P0DSPMQMQuantity (q)$0
第 1 步 · 市场图：画向下倾斜的 D、向上倾斜的 S，标出均衡价格 PM 和数量 QM。市场用大写 Q。
Market 市场Firm 企业Quantity (Q)P0DSPMQMQuantity (q)$0d = MR = AR = PPF
第 2 步 · 把价格拉过来：从 PM 画一条水平线到企业图：这就是企业面对的需求 d = MR = AR = P，标 PF。两张图的价格高度必须一样。
Market 市场Firm 企业Quantity (Q)P0DSPMQMQuantity (q)$0d = MR = AR = PPFMCqF
第 3 步 · 画 MC 找产量：画向上倾斜的 MC，在 MR = MC 处向下读出企业产量 qF（小写 q）。
Market 市场Firm 企业Quantity (Q)P0DSPMQMQuantity (q)$0d = MR = AR = PPFMCqFATC
第 4 步 · 画 ATC：长期均衡：ATC 的最低点正好落在 d = MR 上、位于 qF 处（MC 穿过 ATC 最低点），经济利润为 0。若题目是短期盈利，ATC 在 qF 处低于 P，并涂出利润矩形；短期亏损则 ATC 高于 P。
评分常见扣分点：两张图价格没对齐；企业的需求线没有标 d = MR（或 P = MR）；MC 没有穿过 ATC 最低点；长期均衡时 ATC 最低点没落在价格线上；市场和企业的数量标签混用。

From Short Run to Long Run 从短期到长期（CED PRD-3.A.6）

下面两张图把一个冲击拆成两个阶段来看：先是短期（企业数量不变），再是长期（企业进入）。

Market 市场Firm 企业Quantity (Q)P0D1D2S1$21Q2$15Q1Quantity (q)$0ProfitMCATCd₂ = MR₂d₁ = MR₁8.98
短期：需求增加（D₁ → D₂），企业数量来不及变，价格从 $15 升到 $21，市场数量从 Q₁ 增加到 Q₂。每家企业沿 MC 把产量从 8 增加到约 8.9，P > ATC，赚到经济利润（绿色）。
Market 市场Firm 企业Quantity (Q)P0D2S1S2$15Q3Quantity (q)$0MCATCd = MR8
长期：经济利润吸引新企业进入，市场供给右移（S₁ → S₂），价格回到 $15。每家企业回到 q = 8，经济利润回到 0；市场总量从 Q₁ 增加到 Q₃，增加的部分全部来自新企业。
WORKED EXAMPLE 10健身热潮之后的蛋白粉市场 原创

The market for a generic protein powder is perfectly competitive, is a constant-cost industry, and is in long-run equilibrium. A fitness trend permanently increases demand. Describe what happens to (a) the market price, each firm's output, and each firm's economic profit in the short run, and (b) the number of firms, the market price, each firm's output, and economic profit in the long run.

短期长期（与最初相比）
市场价格↑回到原来的 min ATC（不变）
每家企业产量↑（沿 MC 向上）回到原来的 q（不变）
每家企业经济利润> 0= 0
企业数量不变（来不及进入）↑
市场总产量↑↑（增加得更多）
关键：长期中市场总产量的增加全部来自新企业，而不是老企业多生产。如果是成本递增行业，长期价格会比原来高一些（见本节最后）。

Long-Run Equilibrium & Efficiency 长期均衡与效率

长期均衡条件（CED PRD-3.A.7、3.A.9） P = MR = MC = min ATC
  • Allocative efficiency 配置效率：P = MC。价格代表消费者对最后一单位的边际收益（MB），MC 是生产它的边际成本，MB = MC 意味着生产了“社会最想要”的数量（CED PRD-3.A.5）。
  • Productive efficiency 生产效率：P = min ATC。每家企业都在最低的平均成本处生产，没有浪费资源。
  • Zero economic profit：企业只赚正常利润。
完全竞争的长期均衡同时达到两种效率——这是 Unit 4 比较其他市场结构的基准。
Quantity (q)$0MCATCd = MR = AR = P$15q = 8① P = MC → 配置效率② P = min ATC → 生产效率③ P = ATC → 经济利润 = 0
长期均衡的企业：P = MR = MC = min ATC。三个等号分别对应配置效率、生产效率和零经济利润。
价格是一种信息（CED PRD-3.A.2） 价格同时告诉生产者“消费者对最后一单位值多少钱”（MB），也告诉消费者“生产最后一单位要花多少资源”（MC）。每个人只看价格做决定，就自动实现了 MB = MC，不需要任何人统一指挥——这又一次回到了 Unit 1.2 的 invisible hand。

Long-Run Industry Supply 长期行业供给（CED PRD-3.A.8）

Constant-cost industry 成本不变行业：行业扩张时投入品价格不变，新企业的成本和老企业一样 → 长期价格回到原来的 min ATC → 长期供给曲线水平。例：餐馆、小商品零售。
Increasing-cost industry 成本递增行业：行业扩张把投入品价格抬高（适合种牛油果的土地、熟练工人有限）→ 所有企业的成本曲线上移 → 长期价格更高 → 长期供给曲线向上。最常见。
Decreasing-cost industry 成本递减行业：行业扩张让上游供应商也形成规模经济（例如电动车电池、太阳能板）→ 投入品变便宜 → 长期价格更低 → 长期供给曲线向下。
WORKED EXAMPLE 11牛油果热：成本递增行业的长期 原创

Avocados are produced in a perfectly competitive, increasing-cost industry in long-run equilibrium at a price of $2 per pound. Health trends permanently increase demand. Compare the short-run and long-run effects on price, each grower's costs, the number of growers, and economic profit.

短期长期
价格大幅上涨（例如 $3）回落，但仍高于 $2（例如 $2.4）
每家的成本曲线不变上移：适合种牛油果的土地、农工需求增加，变得更贵
果农数量不变增加
经济利润> 0= 0（在新的、更高的 min ATC 处）
和成本不变行业的区别：长期价格没有回到原来的 $2，所以长期行业供给曲线向上倾斜（真题 2013 Q52 就考这一点）。
Pounds of avocados (Q)Price ($/lb)0D1D2S1S2LRSE₁E₂E₃$210$315$2.418
成本递增行业：E₁ 原来的长期均衡（$2）→ E₂ 需求增加后的短期（$3）→ E₃ 新果农进入后的长期（$2.4）。进入抬高了土地和农工的价格，长期价格回不到 $2。连接 E₁ 和 E₃ 的紫线就是向上倾斜的长期行业供给曲线 LRS。
LAB 4从短期到长期 · Entry and Exit

成本不变行业，开始时处于长期均衡（P = $15，每家企业 q = 8，125 家企业，市场需求 P = 25 − Q/100）。选一个冲击，然后一步一步看市场和单个企业怎么调整。上图是市场，下图是一家典型企业。

练习 Practice 3.7

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。

3.7-12000 AP Micro Q39
In the short run in perfect competition, the industry's demand curve and a firm's demand curve have which of the following slopes?
  • Industry's Demand Curve: Horizontal；Firm's Demand Curve: Downward sloping
  • Industry's Demand Curve: Horizontal；Firm's Demand Curve: Horizontal
  • Industry's Demand Curve: Downward sloping；Firm's Demand Curve: Horizontal
  • Industry's Demand Curve: Downward sloping；Firm's Demand Curve: Downward sloping
  • Industry's Demand Curve: Vertical；Firm's Demand Curve: Horizontal
市场（行业）需求向下倾斜；单个完全竞争企业是价格接受者，面对水平的需求曲线。
3.7-22005 AP Micro Q24
If there are many firms in an industry and each firm's product is indistinguishable from the products of all other firms, the individual firm's demand curve will be
  • upward sloping and different for each firm
  • downward sloping and different for each firm
  • downward sloping and identical for every firm
  • horizontal and different for each firm
  • horizontal and identical for every firm
产品无差别 + 企业众多 → 每家企业都是价格接受者，需求曲线都是同一价格上的水平线。
3.7-32005 AP Micro Q41
A constant-cost, perfectly competitive industry is in long-run equilibrium. If the demand for the good increases, which of the following will occur in the long run?
  • The price will remain unchanged.
  • The price will increase.
  • The price will decrease.
  • Economic profits will increase.
  • Economic profits will decrease.
成本不变行业：需求增加 → 短期价格和利润上升 → 企业进入 → 长期价格回到原来的 min ATC。
3.7-42010 AP Micro Q25
A perfectly competitive firm is currently in long-run equilibrium. Its total revenue is $100,000, and the average total cost of production is $100. Which of the following can be concluded from this information?
  • The firm's marginal cost is $1,000, and its profit is positive.
  • The firm's marginal cost is $1,000, and its profit is zero.
  • The firm's output is 1,000 units, and its profit is negative.
  • The firm's output is 1,000 units, and its profit is zero.
  • The firm's output is 1,000 units, and its profit is positive.
长期均衡 P = ATC = $100，所以 q = 100,000 ÷ 100 = 1,000，经济利润 = 0。
3.7-52012 AP Micro Q23
A constant-cost, perfectly competitive gadget industry is in long-run equilibrium. An increase in the number of consumers of gadgets will most likely result in
  • higher short-run and long-run prices for gadgets
  • reduced short-run profits, followed by the exit of some firms
  • an upward shift in all short-run cost curves, followed by a higher long-run price for gadgets
  • a higher short-run price for gadgets, followed by an increase in the quantity produced
  • a decrease in the demand for gadgets, followed by a decrease in the supply of gadgets
消费者增加 → 需求增加 → 短期价格上升；利润吸引新企业进入，长期产量增加，价格回到原水平（成本不变行业）。
3.7-62013 AP Micro Q39
Assume that olive oil is produced in a constant-cost, perfectly competitive industry, which is currently in long-run equilibrium. If the current price of olive oil is $5 per quart and the demand for olive oil increases, then the price of olive oil will change in which of the following ways in the short run and long run?
  • Short Run: Be more than $5；Long Run: Be more than $5
  • Short Run: Be more than $5；Long Run: Be equal to $5
  • Short Run: Be equal to $5；Long Run: Be equal to $5
  • Short Run: Be less than $5；Long Run: Be less than $5
  • Short Run: Be less than $5；Long Run: Be equal to $5
短期：需求增加使价格高于 $5；长期：企业进入，价格回到 $5（成本不变行业）。
3.7-72013 AP Micro Q52
For a perfectly competitive, increasing-cost industry, an increase in the industry’s demand will lead to which of the following in the long run?
  • An upward shift in each firm’s long-run average cost curve
  • An increase in each firm’s profit
  • A decrease in the price of an input and a decrease in total industry profits
  • A decrease in total industry sales
  • A decrease in total producer surplus and an increase in total consumer surplus
成本递增行业：行业扩张抬高投入品价格 → 每家企业的成本曲线上移 → 长期价格高于原来。
3.7-82015 AP Micro 练习卷 Q26
A firm is producing the allocatively efficient level of output if
  • total revenue is equal to total cost
  • marginal revenue is equal to marginal cost
  • price is equal to average total cost
  • price is equal to marginal cost
  • price is equal to total cost
配置效率：P = MC（价格代表边际收益，MB = MC）。(B) MR = MC 是利润最大化条件。
3.7-92016 AP Micro 练习卷 Q24
For a perfectly competitive firm producing the profit-maximizing quantity, the average total cost is $10 and the average variable cost is $8. If the market price for its product is $10, which of the following is true for the firm?
  • It is sustaining a loss and should shut down.
  • It is earning zero economic profit and will remain in business.
  • Its accounting profits exceed implicit costs.
  • It will temporarily shut down until price rises.
  • It is making an economic profit that will attract other firms to the industry.
P = ATC = $10 → 经济利润为 0（赚取正常利润），而且 P > AVC，继续经营。
3.7-102017 AP Micro 练习卷 Q34
Assume that a perfectly competitive firm is in long-run equilibrium. If industry demand for the product increases, how will this firm’s price, output, and profit change in the short run?
  • Price: Increase；Output: Increase；Profit: Increase
  • Price: Increase；Output: Decrease；Profit: Decrease
  • Price: Increase；Output: Increase；Profit: Decrease
  • Price: No change；Output: Decrease；Profit: Decrease
  • Price: Decrease；Output: Increase；Profit: Increase
行业需求增加 → 市场价格上升 → 企业沿 MC 增产，短期获得经济利润。
3.7-112018 AP Micro 练习卷 Q23
Catering, Inc., which provides catering services in a perfectly competitive market, was maximizing profits at the market price of $22 per meal. The market price has recently increased to $28 per meal. Which of the following short-run adjustments will increase profits for Catering, Inc.?
  • Increasing output until the marginal cost equals the new price
  • Increasing output until the average total cost equals the new price
  • Decreasing output because of the increase in market price
  • Increasing the wage rate paid to its workers
  • Increasing advertising to increase demand for the meals
价格从 $22 涨到 $28：MR 上升，企业应沿着 MC 增产，直到 MC = 新价格。
3.7-122021 AP Micro 国际卷 Q17
Which of the following is true for a perfectly competitive, decreasing-cost industry?
  • The price of inputs will decrease as the number of firms in the industry decreases.
  • The long-run market supply curve will be downward sloping.
  • Firms earn positive economic profit in the long run.
  • Firms do not have a shutdown condition.
  • There are no economies of scale.
成本递减行业：行业扩张让投入品变便宜 → 长期价格更低 → 长期供给曲线向下倾斜。
3.7-132000 AP Micro Q7
Questions 6-7 refer to the graph below, which shows the cost curves for a profit-maximizing, perfectly competitive firm.
QUANTITYPRICE/COSTS0Marginal CostAverage Total CostAverage Variable CostP₁Q₁P₂Q₂CFD
按原题重绘
If marginal revenue is equal to P1, all of the following statements are true EXCEPT:
  • Total revenue will equal total costs.
  • The firm will produce Q1 units of output.
  • The firm will produce the efficient level of output.
  • The firm will earn a normal profit.
  • The firm will increase production in the long run.
MR = P₁ = min ATC：在 Q₁ 处 MR = MC，P = ATC → TR = TC，经济利润为 0（正常利润）；P = MC 且 P = min ATC → 有效率。没有经济利润，长期不会扩大产量 → (E) 是错的。
3.7-142010 AP Micro Q24
Questions 23-24 refer to the graph below, which shows the cost curves of a firm.
QUANTITYPRICE0Short-run Marginal CostShort-run ATCLong-run ATCP₁Q₁
按原题重绘
Which of the following will be true if the firm is in a perfectly competitive market and the price is P1 ?
  • The firm will earn short-run profits but suffer long-run losses.
  • In the long run, existing firms in the industry will produce an output level greater than Q1.
  • In the long run, existing firms will leave the industry.
  • Firms will leave the industry until profits are increased.
  • New firms will be dissuaded from entering the industry, at least until the price increases.
价格 P₁ 下企业在 Q₁ 生产，SRATC < P₁ → 短期有经济利润 → 新企业进入，价格一直降到 LRATC 的最低点。LRATC 最低点在 Q₁ 右边，所以长期每家企业的产量会大于 Q₁。
3.7-152014 AP Micro 练习卷 Q36
Questions 35-36 refer to the graph below for a representative firm in a perfectly competitive, constant-cost industry, which shows the firm’s marginal cost (MC), average total cost (ATC), and average variable cost (AVC).
QUANTITYCOSTS0MCATCAVCP₁Q₁P₂Q₂P₃Q₃P₄
按原题重绘
Which of the following MUST be true in the long run?
  • The equilibrium price will be P2, since that is where marginal cost equals minimum average variable cost.
  • The equilibrium price will be above P3, since firms must make an economic profit to stay in business.
  • If the price is above P2, new firms will enter the industry.
  • If the price is above P3, new firms will enter the industry.
  • If the price is above P4, firms will exit the industry.
长期均衡价格 = min ATC = P₃。价格高于 P₃ → 经济利润 > 0 → 新企业进入。(A) P₂ 是停产点；(B) 长期只赚正常利润；(E) 价格高于 P₄ 有利润，不会退出。
3.7-162015 AP Micro 练习卷 Q39
Questions 39-40 are based on the graph below showing the long-run adjustment of a constant-cost perfectly competitive industry.
OUTPUTPRICE0DD′SS′P₂P₁Q₂PQ₁Q₃RCORN MARKET
按原题重绘
Assume that the corn market is initially in long-run equilibrium at point R. What are the short-run and long-run prices of corn if more corn is used as a source of alternative energy?
  • Short Run: P；Long Run: P1
  • Short Run: P；Long Run: P2
  • Short Run: P1；Long Run: P2
  • Short Run: P2；Long Run: P1
  • Short Run: P2；Long Run: P
玉米用来发电 → 需求从 D 增加到 D′：短期沿着 S 涨到 P₂；利润吸引新企业进入，供给右移到 S′，成本不变行业的长期价格回到 P。
3.7-172015 AP Micro 练习卷 Q40
Questions 39-40 are based on the graph below showing the long-run adjustment of a constant-cost perfectly competitive industry.
OUTPUTPRICE0DD′SS′P₂P₁Q₂PQ₁Q₃RCORN MARKET
按原题重绘
The long-run industry supply curve for corn is
  • upward sloping
  • downward sloping
  • horizontal
  • downward sloping at first and then upward sloping
  • upward sloping at first and then downward sloping
成本不变行业：原来的长期均衡 R 和新的长期均衡 (Q₃, P) 价格相同，连起来是 P 处的水平线。

ΣSummary 本册总结与易错点

3.4 Types of Profit

会计利润 = TR − 显性成本；经济利润 = TR − 显性 − 隐性成本。经济利润 = 0 即正常利润。会计利润 ≥ 经济利润。企业进出看的是经济利润。

3.5 Profit Maximization

所有企业都在 MR = MC（MC 上升段）处生产；表格题做到最后一个 MR ≥ MC 的单位。完全竞争企业 MR = AR = P。总量法：TR 与 TC 的竖直距离最大处。

3.6 Shutdown & Entry/Exit

利润 = (P − ATC) × q*。短期：P ≥ AVC 生产，P < AVC 停产（亏 FC）。停产价 = min AVC，收支相抵价 = min ATC。企业短期供给 = AVC 以上的 MC。长期：利润 → 进入，亏损 → 退出。固定成本变化不改变产量和停产决策。

3.7 Perfect Competition

大量企业、同质产品、自由进出、信息完全 → price taker，d = MR = AR = P。并排图价格对齐。需求增加：短期价格↑、利润 > 0；长期进入，价格回到 min ATC（成本不变）。长期均衡 P = MC = min ATC：配置效率 + 生产效率，经济利润 0。长期行业供给：成本不变水平、成本递增向上、成本递减向下。

决策速查

要回答的问题比较什么规则
生产多少？MR 与 MCMR = MC（表格题做到最后一个 MR ≥ MC 的单位）
赚还是亏？q* 处的 P 与 ATCP > ATC 盈利；P = ATC 正常利润；P < ATC 亏损
短期要不要停产？P 与 AVC（TR 与 TVC）P < min AVC 停产，亏损 = FC
长期进还是出？经济利润（P 与 ATC）> 0 进入；< 0 退出；= 0 长期均衡
有没有效率？P 与 MC、min ATCP = MC 配置效率；P = min ATC 生产效率
易错点 TOP 10
  1. 会计利润 > 0 企业就会留下 ✗ → 看经济利润，会计利润为正时经济利润仍可能为负。
  2. 经济利润为 0 = 不赚钱 ✗ → 赚到了正常利润。
  3. 利润最大化在 ATC 最低处或 TR 最大处 ✗ → 在 MR = MC 处。
  4. MC 与 MR 有两个交点时随便选 ✗ → 选 MC 上升时的那个交点。
  5. 亏损就应该马上停产 ✗ → 短期只要 P ≥ AVC 就继续生产。
  6. 停产后亏损为 0 ✗ → 短期停产仍要承担全部固定成本。
  7. 短期停产看 ATC ✗ → 短期看 AVC，长期进出看 ATC。
  8. 企业短期供给曲线是整条 MC ✗ → 只是 AVC 最低点以上的部分。
  9. 完全竞争企业的需求曲线向下倾斜 ✗ → 企业的需求是水平的，市场需求才向下倾斜。
  10. 长期新增的产量来自老企业增产 ✗ → 成本不变行业中，长期每家企业回到原来的产量，增量全部来自新企业。

✎Unit 3（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。

T12022 AP Micro 国际卷 Q1
If accounting profit is greater than zero for a firm in a competitive industry, then which of the following must be true?
  • The firm has no fixed cost.
  • The firm has positive economic profit.
  • The firm’s total revenue exceeds its explicit cost.
  • The firm’s economic profit exceeds its opportunity cost.
  • New firms will enter the industry in the long run.
会计利润 = TR − 显性成本 > 0 → TR > 显性成本。经济利润还要减去隐性成本，不一定为正。
T22023 AP Micro 国际卷 Q56
Which of the following ignores implicit costs?
  • Normal profit
  • Accounting profit
  • Economic profit
  • Opportunity cost
  • Economic cost
会计利润只减显性成本，忽略隐性成本。
T32021 AP Micro 国际卷 Q49
A perfectly competitive firm finds that for the last shirt that it produces and sells, the marginal revenue is $20 and the marginal cost is $22. The firm should
  • charge $22 to maximize profit
  • produce more shirts to increase profit
  • produce fewer shirts to increase profit
  • hire more workers to increase production
  • shut down if the average variable cost is less than $20
MC（22）> MR（20）→ 减产。完全竞争企业是价格接受者，不能自己定价。
T42000 AP Micro Q54
If a perfectly competitive firm wishes to maximize profits and is producing where price exceeds both marginal cost and average variable cost, then the firm is
  • producing too little output
  • producing where its marginal revenue is less than its marginal cost
  • using too much of the variable input
  • charging too high a price for its output
  • not earning any economic profits
P（= MR）> MC → 增产还能提高利润，现在产量太少。
T52015 AP Micro 练习卷 Q38
In the graph below, TC is total cost and TR is total revenue.
OUTPUT$0TRTCQ₁Q₂Q₃Q₄Q₅G
按原题重绘
At which level of output is profit maximized?
  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
TR 在 TC 上方、竖直距离最大的是 Q₃（这里 TC 的斜率 = TR 的斜率，即 MC = MR）。Q₂、Q₄ 利润为 0；Q₁、Q₅ 亏损。
T62016 AP Micro 练习卷 Q52
A profit-maximizing firm will shut down in the short run any time the firm’s total revenue is less than its
  • total cost
  • fixed cost
  • total variable cost
  • explicit cost
  • implicit cost
短期停产条件：TR < TVC（等价于 P < AVC）。
T72022 AP Micro 国际卷 Q59
In the short run, when a profit-maximizing, perfectly competitive firm is producing at the quantity where MR=MC and the firm’s total revenue is less than its total variable cost, the firm should
  • continue to produce the same quantity
  • shut down
  • lower its price
  • raise its price
  • expect other firms to enter the industry
TR < TVC → 生产比停产亏得更多 → 停产。
T82025 AP Micro 国际卷 Q36
QuantityPrice, Cost0Marginal CostAverage Total CostAverage Variable CostP₁P₂P₃Q₁Q₂Q₃Q₄
按原题重绘
The graph shows the cost and revenue curves for the firm Emma’s Hoops.

Emma’s Hoops produces toy hoops that are identical to all other toy hoops produced by the many firms in a market with no barriers to entry. The market price of toy hoops is P2, and Emma’s Hoops is producing the quantity Q2 of toy hoops. Which of the following is true?
  • The firm’s average fixed cost is equal to P3 − P1.
  • The firm is currently earning negative economic profit.
  • The firm is facing a perfectly inelastic demand curve.
  • The firm should decrease production to Q1 to maximize economic profit.
  • The firm should increase its price to P3 to maximize economic profit.
价格 P₂ 下，Q₂ 处 MR = MC，此处 ATC 高于 P₂ → 经济利润为负（P₂ > P₁ = min AVC，仍继续生产）。(A) P₃ − P₁ 是两个不同产量处的最低点之差，不是 AFC；(C) 企业面对完全有弹性的需求；(D) Q₂ 已经是利润最大；(E) price taker 不能提价。
T92019 AP Micro 练习卷 Q51
Units of Output$0MCATCAVCQ₁RQ₂SQ₃TQ₄WQ₅V
按原题重绘
The graph above shows the cost curves facing May’s Fruit Farm, where MC is marginal cost, ATC is average total cost, and AVC is average variable cost. May’s short-run supply curve includes which of the following points?
  • TW
  • RST
  • STV
  • STW
  • RSTV
短期供给 = MC 在 AVC 最低点及以上的部分：S（AVC 最低点）、T、V。R 在 AVC 最低点以下；W 在 ATC 上，不在 MC 上。
T102016 AP Micro 练习卷 Q12
Which of the following is true of a perfectly competitive firm in long-run equilibrium?
  • It produces its output at minimum average total cost.
  • It earns positive economic profits.
  • It will exit the industry.
  • Its price exceeds marginal cost.
  • Its price exceeds marginal revenue.
长期均衡 P = MC = min ATC，经济利润为 0，在最低平均成本处生产（生产效率）。
T112018 AP Micro 练习卷 Q27
A profit-maximizing, perfectly competitive firm is currently in long-run equilibrium. It is earning $15,000 of total revenue from a sale of 1,000 units. Its total fixed cost of production is $2,500. Which of the following can correctly be inferred from the information provided?
  • Its marginal cost is $12.50, and its average total cost is $12.50.
  • Its marginal cost is $12.50, and its average variable cost is $12.50.
  • Its marginal cost is $15.00, and its average total cost is $12.50.
  • Its marginal cost is $15.00, and its average variable cost is $12.50.
  • Its marginal cost is $15.00, and its average fixed cost is $12.50.
长期均衡：P = 15,000 ÷ 1,000 = $15 = MR = MC = ATC。AFC = 2,500 ÷ 1,000 = 2.5，所以 AVC = 15 − 2.5 = $12.50。
T122019 AP Micro 练习卷 Q53
If individual firms in a perfectly competitive market are earning positive economic profits, the number of firms and the price of the product in the market will most likely change in which of the following ways in the long run?
  • Number of Firms: Increase；Price: Increase
  • Number of Firms: Decrease；Price: Increase
  • Number of Firms: Increase；Price: Decrease
  • Number of Firms: Decrease；Price: Decrease
  • Number of Firms: No change；Price: Decrease
正的经济利润吸引企业进入 → 市场供给增加 → 价格下降，直到经济利润为 0。
T132022 AP Micro 国际卷 Q41
Assume paper clips are produced in a perfectly competitive industry that is currently in long-run equilibrium. One firm in the industry develops a new production method that reduces its variable cost of production. If its competitors are unable to copy this new method, what will happen to this firm’s output and profit in the short run?
  • Output will increase, and profit will increase.
  • Output will decrease, and profit will increase.
  • Output will not change, and profit will increase.
  • Output will increase, and profit will not change.
  • Output will not change, and profit will not change.
这家企业的 MC 下降，而市场价格不变 → 在更大的产量处 MR = MC → 产量增加、利润增加（其他企业学不会，暂时独享这个优势）。
T142023 AP Micro 国际卷 Q13
Donuts for Dollars is a typical profit-maximizing firm in a perfectly competitive market that is earning positive economic profit in the short run. Which of the following statements must be true?
  • Price must be less than average total cost in the short run, and firms will exit the market in the long run.
  • Price must be less than average total cost in the short run, and firms will earn normal profit in the long run.
  • Price must equal average total cost in the short run, and firms will enter the market in the long run.
  • Price must be greater than average total cost in the short run, and firms will exit the market in the long run.
  • Price must be greater than average total cost in the short run, and firms will earn normal profit in the long run.
有经济利润 → P > ATC；企业进入使价格下降，长期只赚正常利润。
T152025 AP Micro 国际卷 Q28
Firm Z is a typical firm in a perfectly competitive market that produces a quantity of output where marginal revenue equals marginal cost in a constant-cost industry. If the sum of Firm Z’s average fixed cost and average variable cost is below its marginal revenue at the profit-maximizing quantity, which of the following will occur in the long run?
  • The firm will increase output.
  • The firm will shut down.
  • The firm will increase price.
  • The firm will exit the market.
  • New firms will enter the market.
AFC + AVC = ATC < MR = P → 企业有经济利润 → 长期新企业进入。
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FRQFree-Response Practice 自由问答

前两道是 AP 真题，评分标准依照 College Board 官方 Scoring Guidelines 整理；最后一道是原创的停产综合题。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 Q3 · 5 分 · 完全竞争企业图 真题

The graph provided shows the demand (d), marginal revenue (MR), average total cost (ATC), average variable cost (AVC), and marginal cost (MC) curves for Hansel Hangout, a typical profit-maximizing firm in a perfectly competitive market producing Good X.

QuantityPrice, Cost ($)0MCd = MRAVCATC26421620889111410
按原题重绘（刻度：ATC 在 q = 4、6、8 时分别为 26、21、20；AVC 分别为 8、9、11；d = MR 在 $14）

(a) Calculate Hansel Hangout's total fixed cost. Show your work.
(b) Identify the price and Hansel Hangout's profit-maximizing quantity of Good X.
(c) Calculate Hansel Hangout's economic profit at the quantity identified in part (b). Show your work.
(d) As the market for Good X adjusts to the long-run equilibrium, what will happen to the price of Good X? Explain.
(e) Assume the cross-price elasticity of demand between Good X and Good C is positive. Given the change in the long-run price of Good X in part (d), will the quantity demanded of Good C increase, decrease, or remain the same? Explain.

Point得分要求与解析
(a) 1 ptTFC = (ATC − AVC) × q = (26 − 8) × 4 = $72（用 q = 6 或 8 的数据也一样：(21 − 9) × 6、(20 − 11) × 8）。
(b) 1 ptP = $14，q = 6（MR = MC）。
(c) 1 pt利润 = (14 − 21) × 6 = −$42。注意：P = 14 > AVC = 9，短期继续生产。
(d) 1 pt价格上升：负的经济利润使部分企业退出，市场供给左移，价格上升（直到 min ATC = $20）。
(e) 1 ptGood C 的需求量增加：XED 为正 → 替代品；X 涨价使 C 的需求增加，C 的需求量随之增加。
FRQ 2 · 2025 AP Micro FRQ Set 2 Q1（长题）· 10 分 · 长期均衡、固定成本、补贴、规模 真题

Deskward is a typical profit-maximizing firm that produces and sells wooden desks in a constant-cost, perfectly competitive market that is in long-run equilibrium.

A. Draw correctly labeled side-by-side graphs for the wooden desk market and for Deskward and show each of the following.
  i. The market equilibrium price and quantity, labeled PM and QM, respectively
  ii. Deskward's profit-maximizing price and quantity, labeled PF and QF, respectively
  iii. Deskward's average total cost curve consistent with long-run equilibrium, labeled ATC
B. If the monthly rent, a fixed cost, on Deskward's factory building increases, what will happen to the firm's profit-maximizing quantity in the short run? Explain.
C. Suppose the government is considering granting a per-unit subsidy to producers of wooden desks. On your market graph in part A, show the short-run effect of a per-unit subsidy on each of the following.
  i. The new market equilibrium price and quantity of wooden desks, labeled P* and Q*, respectively
  ii. The area representing the total cost of the subsidy to the government, shaded completely
D. Instead of the per-unit subsidy, suppose the government imposes a binding price floor in the market for wooden desks. Will the price floor result in a shortage of wooden desks, a surplus of wooden desks, or neither? Explain.
E. Deskward also produces chairs. Deskward increases its production from 500 chairs to 600 chairs, and its long-run total cost increases from $80,000 to $108,000.
  i. Calculate Deskward's long-run average total cost of producing 500 chairs. Show your work.
  ii. As Deskward increases production from 500 chairs to 600 chairs, is Deskward experiencing economies of scale, diseconomies of scale, or the efficient scale? Explain using numbers.

Point得分要求与解析
A 1 pt市场图：向下的 D、向上的 S，标出 PM、QM。
A 1 pt企业图：从 PM 水平延伸过来的 d = MR，标为 PF。
A 1 pt企业图有向上的 MC，在 MR = MC 处标出 QF。
A 1 ptATC 在 QF 处与 d = MR 相切（长期均衡），MC 穿过 ATC 最低点。
B 1 pt产量不变：固定成本变化不影响 MC 和 MR。
C 1 pt市场供给右移，标出新的 P*（更低）、Q*（更多）。
C 1 pt完整涂出补贴总成本：每单位补贴（新供给与原供给在 Q* 处的竖直距离）× Q* 的矩形。
D 1 ptSurplus：有效的价格下限高于均衡价格，供给量大于需求量。
E(i) 1 ptLRATC = 80,000 ÷ 500 = $160。
E(ii) 1 ptDiseconomies of scale：LRATC 从 $160 升到 108,000 ÷ 600 = $180。
Market 市场Firm 企业Quantity (Q)P0补贴总成本DS1S2PMQMP*Q*Quantity (q)$0MCATCd = MRPFqF
参考图。市场：D 与 S₁ 决定 PM、QM（A 部分）。补贴使供给右移到 S₂，新均衡 P*、Q*；黄色矩形 = 每单位补贴（S₁ 与 S₂ 在 Q* 处的竖直距离）× Q*（C 部分）。企业：d = MR 从 PM 水平拉过来，标 PF；qF 在 MR = MC 处；长期均衡时 ATC 最低点切在 d = MR 上（A 部分）。
FRQ 3 · 综合练习 · 9 分 · 梨园：亏损、停产与长期退出 原创

Pear Orchard is a typical profit-maximizing firm in a perfectly competitive, constant-cost industry. Its fixed cost is $50. The table shows its total cost.

Output01234567
Total cost ($)507490102118140170210

The market price of pears is $24.
(a) Identify Pear Orchard's profit-maximizing quantity. Explain using marginal analysis.
(b) Calculate Pear Orchard's economic profit at that quantity. Show your work.
(c) In the short run, should Pear Orchard produce or shut down? Explain using numbers.
(d) Draw correctly labeled side-by-side graphs for the pear market and for Pear Orchard. On your graphs, show (i) the market price PM and Pear Orchard's price PF; (ii) Pear Orchard's marginal cost curve and profit-maximizing quantity qF; (iii) Pear Orchard's average total cost curve consistent with part (b), and completely shade the area representing its economic profit or loss.
(e) In the long run, what will happen to the number of firms in the pear industry? Explain.
(f) Identify the long-run equilibrium price of pears. Explain.
(g) Suppose instead that the market price falls to $16 in the short run. How many pears will Pear Orchard produce, and what will its economic profit be?

Output1234567
MC24161216223040
AVC242017.3317182022.86
ATC74453429.52828.3330
Profit at P = $24−50−42−30−22−20−26−42
Point得分要求与解析
(a) 1 pt5 个。MR = P = $24：第 5 个的 MC = $22 ≤ 24，值得生产；第 6 个的 MC = $30 > 24，不生产。
(b) 1 pt利润 = TR − TC = 24 × 5 − 140 = −$20（或 (P − ATC) × q = (24 − 28) × 5）。
(c) 1 pt继续生产：TR = $120 > TVC = 140 − 50 = $90（P = 24 > AVC = 18）。生产亏 $20，停产要亏全部固定成本 $50。
(d) 1 pt市场图有向下的 D、向上的 S，标出 PM；企业图的 d = MR 从 PM 水平拉过来，标 PF。
(d) 1 pt企业图有向上的 MC，在 MR = MC 处标出 qF。
(d) 1 ptqF 处 ATC 高于 PF，并完整涂出 PF 到 ATC、宽为 qF 的亏损矩形。
(e) 1 pt企业数量减少：经济利润为负，长期中部分企业退出行业。
(f) 1 pt$28：退出使市场供给左移、价格上升，直到经济利润为 0，即价格等于 min ATC = 140 ÷ 5 = $28（成本不变行业，成本曲线不移动）。
(g) 1 pt产量 0，经济利润 −$50：P = $16 低于 min AVC = $17（q = 4），无论生产多少都收不回可变成本，停产只亏固定成本。（如果生产 4 个：TR = 64 < TVC = 68，亏 $54。）
Market 市场Firm 企业Quantity (Q)P0DSPMQMQuantity (q)$0LossMCATCAVCd = MRPFqF
(d) 参考图：两张图价格对齐；qF 在 MR = MC 处；qF 处 ATC 高于 P，红色矩形（P 到 ATC、宽 qF）就是亏损；P 仍高于 AVC。
FRQ 答题提醒
  • “Side-by-side graphs”：市场图和企业图的价格要对齐，企业的 d = MR 从市场价格水平拉过来；市场用大写 Q，企业用小写 q（或题目指定的标签）。
  • 企业图的 MC 必须穿过 ATC（和 AVC）的最低点；长期均衡时 ATC 最低点切在 d = MR 上。
  • 利润/亏损面积：在 q* 处向上找 ATC，P 与 ATC 之间的矩形，要完整涂满。
  • “Explain using marginal analysis” 要写出 MR 与 MC 的比较；停产题要写出 P 与 AVC（或 TR 与 TVC）的比较。
  • 长期调整题按“短期 → 利润或亏损 → 进入或退出 → 供给移动 → 价格回到 min ATC”的顺序写。
回顾 · 上册 本册用到的成本曲线都来自上册：MC、AVC、ATC 的形状与关系，固定成本和每单位成本怎样移动曲线。
→ 打开 Unit 3（上）Production and Costs（与本文件放在同一文件夹）

JL · AP Microeconomics · Unit 3（下）Profit and Perfect Competition · 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 · 原创 依照 AP 题型原创