JL · AP MICROECONOMICS

Unit 2 · Supply and Demand

供给与需求 — 需求、供给、三种弹性、均衡与剩余、均衡变动、政府干预、国际贸易 · 对应 CED 2.1–2.9 · 选择题占比约 20–25%（全书最大的单元）

本单元学习目标 LEARNING OBJECTIVES

区分“需求/供给的变化”和“需求量/供给量的变化”，判断事件让曲线怎样移动 · Distinguish shifts of curves from movements along them
用中点法计算 PED、PES、YED、XED，并用弹性判断总收益变化和商品类型 · Calculate and interpret elasticities; apply the total revenue test
找出市场均衡，计算消费者剩余、生产者剩余和总剩余 · Find equilibrium and calculate CS, PS, total surplus
分析单条或两条曲线移动后价格、数量和剩余的变化 · Analyze single and double shifts
分析价格上限/下限、税、补贴、配额对市场的影响，计算税收、税负和无谓损失 · Analyze government intervention and deadweight loss
分析开放贸易、关税和进口配额对价格、数量和剩余的影响 · Analyze trade, tariffs and quotas

Term 术语	Meaning 含义	中文
Demand / Quantity Demanded	The whole schedule (curve) / the amount at one specific price	需求 / 需求量
Law of Demand	Price ↑ → quantity demanded ↓, other things equal	需求定律
Substitution / Income Effect	Why demand slopes down: switching to relatively cheaper goods / change in purchasing power	替代效应 / 收入效应
Normal / Inferior Good	Demand rises / falls when income rises	正常品 / 劣等品
Substitutes / Complements	Goods used in place of / together with each other	替代品 / 互补品
Supply / Quantity Supplied	The whole schedule (curve) / the amount at one specific price	供给 / 供给量
Price Elasticity of Demand (PED)	%ΔQd ÷ %ΔP — responsiveness of buyers to price	需求价格弹性
Elastic / Inelastic / Unit Elastic	\|E\| > 1 / \|E\| < 1 / \|E\| = 1	富有弹性 / 缺乏弹性 / 单位弹性
Total Revenue (TR)	P × Q; equals consumers' total expenditure	总收益
Price Elasticity of Supply (PES)	%ΔQs ÷ %ΔP	供给价格弹性
Income Elasticity (YED)	%ΔQd ÷ %ΔIncome; sign shows normal vs inferior	收入弹性
Cross-Price Elasticity (XED)	%ΔQd of A ÷ %ΔP of B; sign shows substitutes vs complements	交叉价格弹性
Equilibrium	Price at which Qd = Qs	均衡
Shortage / Surplus	Qd > Qs (price too low) / Qs > Qd (price too high)	短缺 / 过剩
Consumer / Producer Surplus	WTP − price / price − seller's cost	消费者剩余 / 生产者剩余
Total Economic Surplus	CS + PS (+ government revenue when there is a tax)	总剩余
Deadweight Loss (DWL)	Lost total surplus when quantity ≠ efficient quantity	无谓损失
Price Ceiling / Price Floor	Legal maximum / minimum price	价格上限 / 价格下限
Per-Unit (Excise) Tax / Subsidy	Fixed amount taken / given per unit sold	从量税 / 补贴
Tax Incidence	How the burden of a tax is divided between buyers and sellers	税负归宿
Quota	Legal limit on quantity (produced or imported)	配额
World Price / Tariff	Price in the world market / a tax on imports	世界价格 / 关税

考试结构 EXAM FORMAT Unit 2 在选择题里大约 12–15 道，是全卷最多的单元；FRQ 几乎每年都有一道以供需图为主的题（价格管制、税、剩余计算、弹性、贸易），长题里的“关联市场”也常从这里出。画图的标准：纵轴 Price，横轴 Quantity，曲线标 D、S，原均衡标 P₁、Q₁，新均衡标 P₂、Q₂，并用箭头表示移动方向。

2.1Demand 需求

为什么奶茶店喜欢搞“第二杯半价”？为什么“双十一”降价时大家疯狂下单？为什么油价一涨，电动车就卖得更好？这一节用一条向右下倾斜的线——需求曲线——把这些现象统一解释。

定义 DEFINITION Demand: the quantities of a good that buyers are willing and able to buy at each possible price during a period of time, other things held constant (ceteris paribus).
Quantity demanded: the amount buyers want to buy at one specific price.

注意两个条件缺一不可：想买（willing）并且买得起（able）。你很想要一辆跑车，但买不起，就不算需求。

Demand Schedule & Demand Curve 需求表与需求曲线

Lily 每周买多少杯奶茶，取决于一杯的价格：

Price per cup	$2	$3	$4	$5	$6
Quantity demanded (cups/week)	8	6	4	3	2

把需求表的每一行画成一个点，连起来就是需求曲线：价格越高，需求量越少（向右下倾斜）

Law of Demand 需求定律 Other things equal, when the price of a good rises, the quantity demanded falls (and vice versa). 价格与需求量反方向变化，所以需求曲线向右下倾斜。
AP 惯例：价格（P）画在纵轴，数量（Q）画在横轴。需求曲线也可以理解为边际收益（MB）曲线：每一点的高度，就是消费者愿意为“再多一单位”付的最高价（衔接 Unit 1.6）。

Why Does Demand Slope Downward? 为什么向右下倾斜

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Substitution effect 替代效应：奶茶涨价，相比之下果汁、咖啡变得“便宜”了 → 一部分人改买别的 → 奶茶需求量↓。

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Income effect 收入效应：奶茶涨价，同样的零花钱能买的东西变少了（实际购买力下降）→ 买得更少。

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Diminishing marginal utility 边际效用递减：每多一杯带来的满足感越来越小，所以只有价格更低时，你才愿意买更多杯（Unit 1.6）。

Individual vs. Market Demand 个人需求与市场需求

市场需求是所有买家需求的加总（CED MKT-3.A.6）。在每一个价格下，把所有人的需求量相加：

Price	Amy	Ben	Market
$6	1	0	1
$5	2	1	3
$4	3	2	5
$3	4	3	7
$2	5	4	9

市场需求 = 每个价格下把所有人的需求量横向相加（价格不加，数量相加）

Movement Along vs. Shift of the Curve 沿曲线移动 vs. 曲线平移

这是整个 Unit 2 最重要的区分，后面所有的图都建立在它上面。

沿着曲线移动：只有商品自身价格变了（P₁ → P₂），需求量从 Q₁ 变到 Q₂。这叫 change in quantity demanded。

整条曲线平移：价格没变（都是 P），但同样的价格下人们想买得更多了。这叫 change in demand（需求增加 = 曲线右移）。

易错 TRAP

商品自身价格变化永远只会让点沿着曲线移动，不会让需求曲线平移。题目说“the price of X rises, so the demand for X decreases”是错的——应该说 quantity demanded decreases。
“Demand increases” = 整条曲线右移；“quantity demanded increases” = 沿曲线向右下移动。考试会故意混用这两个说法。

Determinants of Demand 需求的决定因素（让 D 平移的原因）

除了自身价格，其他会改变需求的因素可以记成 T-R-I-B-E：

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Tastes and preferences 偏好：网红带货、健康新闻、潮流变化。偏好↑ → D 右移。

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Related goods' prices 相关商品价格：
• Substitutes 替代品（可乐 vs 百事）：替代品涨价 → 本商品 D 右移。
• Complements 互补品（打印机 vs 墨盒、汽车 vs 汽油）：互补品涨价 → 本商品 D 左移。

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Income 收入：
• Normal good 正常品：收入↑ → D 右移（大多数商品，如旅游、外卖）。
• Inferior good 劣等品：收入↑ → D 左移（如二手衣服、公交车、最便宜的方便面——有钱以后会换成更好的）。

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Buyers 买家数量：人口增长、新市场开放、旅游旺季 → D 右移。

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Expectations 预期：预期明天会涨价 → 今天抢着买 → 今天的 D 右移；预期会降价 → 先等等 → 今天的 D 左移。预期收入上升也会让需求增加。

考纲细节 · 激励与约束（CED MKT-3.A.1–3）市场能正常运转需要明确的产权（property rights）；人们会对激励（比如价格）做出反应，但同时受到约束：收入、时间、法律法规。例如很多人想买房，但受首付（收入约束）和限购政策（法规约束）限制。

WORKED EXAMPLE 1六个事件：电动车的需求怎么变？原创

For each event, decide whether the demand curve for electric vehicles (EVs) shifts right, shifts left, or whether there is a movement along the demand curve.

汽油价格大涨
居民收入普遍提高
政府宣布：明年起电动车购置补贴取消
电动车自身降价 10%
城市里新建了大量充电桩
电池起火的新闻在网上刷屏

1. 汽油价格大涨
汽油是燃油车的“搭档”，燃油车用起来更贵 → 人们转向电动车。汽油车与电动车是替代品：替代品涨价 → D 右移。

2. 居民收入普遍提高
电动车是 normal good（正常品）：收入↑ → D 右移。

3. 政府宣布：明年起电动车购置补贴取消
预期未来会变贵 → 大家赶在今年买 → 今年的 D 右移。

4. 电动车自身降价 10%
自身价格变化 → 不是平移，而是沿着 D 向右下移动（需求量增加）。

5. 城市里新建了大量充电桩
充电桩与电动车是互补品，充电更方便、成本更低 → D 右移。

6. 电池起火的新闻在网上刷屏
偏好变差 → D 左移。

判断步骤：① 是不是这个商品自己的价格变了？是 → 沿曲线移动。② 不是 → 属于 TRIBE 中的哪一项？→ 判断方向。

练习 Practice 2.1

以下为历年 AP 真题，点选项即时判分并显示解析。

2.1-12000 AP Micro Q5

Which of the following events will cause the demand curve for hamburgers to shift to the right?

An increase in the price of pizza, a substitute for hamburgers
An increase in the price of french fries, a complement to hamburgers
An increase in the price of hamburgers
A decrease in the price of hamburgers
A decrease in the cost of producing hamburgers

披萨是汉堡的替代品，披萨涨价 → 人们改吃汉堡 → 汉堡需求右移。(B) 互补品涨价让需求左移；(C)(D) 是自身价格，只沿曲线移动；(E) 影响供给。

2.1-22005 AP Micro Q5

Which of the following will decrease the demand for beef?

An increase in the price of pork, if pork and beef are substitute goods
An increase in the price of potatoes, if potatoes and beef are complementary goods
A decrease in the cost of transporting beef to consumers
An increase in the income levels of most consumers, if beef is a normal good
Research showing beef is better for your health than chicken

土豆是牛肉的互补品，土豆涨价 → 牛肉需求减少。(A) 替代品涨价让需求增加；(C) 运输成本影响供给；(D)(E) 都让需求增加。

2.1-32013 AP Micro Q5

According to the law of demand, an increase in the price of grape juice will result in

a rightward shift in the demand curve for grape juice
a leftward shift in the demand curve for grape juice
a decrease in the demand for orange juice, a substitute
a decrease in the quantity of grape juice demanded
an increase in the quantity of grape juice supplied

需求定律说的是价格与需求量反向变化：沿着需求曲线移动，曲线本身不动。

2.1-42013 AP Micro Q49

Which of the following explains why a decrease in the price of a normal good will lead to an increase in the quantity demanded of the good?

A lower price will increase consumers’ purchasing power.
A lower price will increase consumers’ marginal utility.
A lower price will increase demand for the good.
A lower price will increase demand for substitute goods.
A lower price will decrease demand for complementary goods.

降价让消费者的实际购买力上升（收入效应），所以买得更多。(C) 降价不会让“需求”增加，只是需求量增加。

2.1-52016 AP Micro 练习卷 Q34

For an inferior good, an increase in consumer income will cause

the demand curve to shift to the left
the demand curve to shift to the right
the short-run supply curve to shift to the right
the long-run supply curve to shift to the right
new firms to enter the market in the long run

劣等品：收入增加 → 需求减少 → 需求曲线左移。收入影响的是需求，不是供给。

2.1-62017 AP Micro 练习卷 Q6

Which of the following will increase the demand for pizza, a normal good?

An increase in the cost of producing pizza
A decrease in the price of pizza
An increase in the price of a complementary product
An increase in consumers’ income
An increase in the number of restaurants selling pizza

披萨是正常品，收入增加 → 需求右移。(A)(E) 影响供给；(B) 是自身价格；(C) 互补品涨价让需求左移。

2.1-72018 AP Micro 练习卷 Q19

All of the following cause a rightward shift in the demand schedule for a normal good EXCEPT

a decrease in the price of the good
a decrease in the price of a complementary good
an increase in the price of a substitute good
an increase in consumers’ income
an increase in consumers’ preference for the good

自身降价只是沿需求曲线移动（需求量增加），不是需求曲线右移。其余四项都让需求右移。

2.1-82017 AP Micro 练习卷 Q13

The table below lists the monthly individual demand schedules for text messages for the only three buyers in the market: Allison, Bill, and Chang.

Price per text message	Allison	Bill	Chang
4¢	500	400	360
8¢	350	300	280
12¢	200	200	200
16¢	50	100	120
20¢	0	0	40

Which of the following combinations of price and quantity lies on the market demand curve?

Price: 4¢；Quantity: 1,000
Price: 8¢；Quantity: 350
Price: 12¢；Quantity: 200
Price: 16¢；Quantity: 270
Price: 20¢；Quantity: 0

市场需求 = 同一价格下三人需求量之和。16¢：50 + 100 + 120 = 270 ✓。其他：4¢ 为 1,260，8¢ 为 930，12¢ 为 600，20¢ 为 40。

2.1-92023 AP Micro 国际卷 Q4

A decrease in the demand for antique lanterns, a normal good, would be caused by which of the following?

An increase in consumers’ incomes
An increase in the price of antique lanterns
An increase in the expected price of antique lanterns in the future
An increase in the price of electric lanterns, a substitute good
An increase in the price of lamp oil, a complementary good

灯油是互补品，灯油涨价 → 古董灯的需求减少。(A)(C)(D) 都让需求增加；(B) 自身价格只影响需求量。

2.2Supply 供给

需求说的是买家，供给说的是卖家。草莓价格涨到很高时，农场会多雇人去摘、甚至把别的地也改种草莓；价格很低时，可能连摘都不值得。

定义 DEFINITION Supply: the quantities of a good that sellers are willing and able to sell at each possible price during a period of time, ceteris paribus.
Law of supply: other things equal, a higher price leads to a higher quantity supplied. 价格与供给量同方向变化 → 供给曲线向右上倾斜（CED MKT-3.C.1）。

Price per box	$2	$3	$4	$5	$6
Quantity supplied (boxes/day)	1	3	5	6	7

供给曲线向右上倾斜：草莓价格越高，农场愿意摘、愿意卖的越多

为什么向右上倾斜？ ① 利润激励：价格越高，卖得越多越赚钱，也能吸引原本做别的生意的资源进来。② 边际成本递增：多生产一单位要用上越来越“不合适”的资源（加班、雇不熟练的工人），成本越来越高，所以只有价格更高时卖家才愿意多供给。供给曲线也可以理解为边际成本（MC）曲线（Unit 3 会详细讲）。市场供给同样是所有卖家供给的横向加总（MKT-3.C.2）。

Determinants of Supply 供给的决定因素（让 S 平移的原因）

可以记成 R-O-T-T-E-N：

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Resource (input) prices 投入品价格：原材料、工资、租金↑ → 成本↑ → S 左移。

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Other goods' prices 其他可生产商品的价格：农民的地既能种玉米也能种大豆。大豆涨价 → 农民改种大豆 → 玉米的 S 左移。

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Technology 技术：技术进步 → 成本↓ → S 右移。

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Taxes & subsidies 税与补贴：对卖家征税 → S 左移（向上移，移动距离 = 税额）；补贴 → S 右移。

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Expectations 卖家预期：预期未来价格上涨 → 现在囤货少卖 → 当前 S 左移。

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Number of sellers 卖家数量：新企业进入 → S 右移；企业倒闭退出 → S 左移。另外，天气、自然灾害对农产品供给影响很大。

供给增加 = 供给曲线右移。因为曲线向上倾斜，右移在图上看起来也是“向下”移：同样的数量，卖家愿意接受更低的价格。

易错 TRAP

“供给增加”在图上是向右（看起来向下）移动；“供给减少”是向左（看起来向上）移动。不要因为“增加”就往上画。
需求增加会让价格上涨，但价格上涨不会让供给曲线移动——只会沿着供给曲线让供给量增加。
消费者收入、偏好的变化影响的是需求，不是供给。

WORKED EXAMPLE 2“鸡蛋自由”是怎么没的？——六个事件看供给原创

For each event, decide whether the supply curve of eggs shifts right, shifts left, or whether there is a movement along the supply curve.

饲料（玉米、豆粕）涨价
自动化喂养、集蛋设备普及
禽流感导致大量蛋鸡被扑杀
政府给每箱鸡蛋发 $1 补贴
养殖户预期下个月蛋价大涨
鸡蛋价格上涨

1. 饲料（玉米、豆粕）涨价
投入品价格↑ → 每只鸡蛋的成本↑ → S 左移。

2. 自动化喂养、集蛋设备普及
技术进步 → 成本↓ → S 右移。

3. 禽流感导致大量蛋鸡被扑杀
卖家（产能）减少 / 自然冲击 → S 左移。

4. 政府给每箱鸡蛋发 $1 补贴
补贴相当于降低成本 → S 右移。

5. 养殖户预期下个月蛋价大涨
卖家预期未来价格↑ → 现在少卖、囤货 → 当前 S 左移。

6. 鸡蛋价格上涨
自身价格变化 → 沿着 S 向右上移动（供给量增加），曲线不动。

LAB 1平移还是沿曲线移动？ · Shift or Movement?

市场：

每个事件选一个答案，选完立刻出现对应的小图和理由。（得分：0 / ）

练习 Practice 2.2

以下为历年 AP 真题，点选项即时判分并显示解析。

2.2-12000 AP Micro Q48

The supply curve for automobiles will shift to the left in response to

an increase in the efficiency of robot technology
an increase in wages in the automobile industry
a decrease in the number of consumers purchasing automobiles
a decrease in the interest rates for automobile loans
a decrease in consumers' income

汽车行业工资上涨 = 投入成本上升 → 供给左移。(A) 技术进步让供给右移；(C)(D)(E) 都是需求方面的因素。

2.2-22010 AP Micro Q34

Assume that apple juice and grape juice are substitutes. Which of the following will cause an increase in the quantity of apple juice supplied?

A decrease in subsidies to apple juice producers
A decrease in the price of grape juice
An increase in the price of apple juice
An increase in the price of insecticides used on apple trees
The imposition of a price ceiling in the market for apple juice

题目问的是“供给量（quantity supplied）”增加：只有苹果汁自身价格上涨，才会沿供给曲线增加供给量。(A)(D) 让供给减少；(B) 影响需求；(E) 价格上限会减少供给量。

2.2-32013 AP Micro Q48

An increase in the supply of coffee could be caused by

a decrease in the price of cream, which is a complement to coffee
a decrease in the cost of labor used to produce coffee
an increase in consumer income
an increase in the demand for coffee
an increase in the price of coffee

劳动成本下降 → 生产成本下降 → 供给增加。(A)(C)(D) 是需求方面；(E) 只是供给量增加。

2.2-42014 AP Micro 练习卷 Q32

Which of the following will cause the supply curve for shoes to shift to the right?

An increase in the price of socks, assuming that shoes and socks are complements
A decrease in the price of sandals, assuming that shoes and sandals are substitutes
An increase in the wages of shoe workers
An increase in the number of firms producing shoes
A decrease in the income of consumers, assuming that shoes are normal goods

生产鞋的企业增加 → 卖家数量增加 → 供给右移。(A)(B)(E) 影响需求；(C) 工资上涨让供给左移。

2.2-52015 AP Micro 练习卷 Q20

Which of the following would shift the short-run supply curve for strawberries?

A decrease in the consumption of strawberries
An increase in the price of strawberries
A strike by all farmworkers
An increase in household incomes
An announcement of a study that shows the health benefits of eating strawberries

农场工人罢工减少了可用的劳动投入 → 草莓供给左移。其余选项要么是需求因素，要么是自身价格。

2.2-62023 AP Micro 国际卷 Q54

A decrease in the cost of land devoted for building townhouses will most likely cause which of the following?

The supply curve of townhouses will shift to the left.
The demand curve for townhouses will become more elastic.
The price of townhouses will increase.
The supply of townhouses will increase.
There will be less incentive to build townhouses.

土地成本下降 → 建联排别墅更便宜 → 供给增加（右移）。

2.2-72025 AP Micro 国际卷 Q55

Which of the following would cause an increase in the supply of tacos in the short run?

The price of tacos decreases.
The demand for tacos increases.
The price of beef, an input in the production of tacos, decreases.
The price of guacamole, a complement in consumption for tacos, decreases.
The price of burritos, a substitute in consumption for tacos, decreases.

牛肉是做塔可的投入品，投入品降价 → 塔可供给增加。(A) 自身价格；(B)(D)(E) 是需求方面。

2.3Price Elasticity of Demand 需求的价格弹性

需求定律只告诉我们“涨价会让需求量下降”，却没说降多少。汽油涨 10%，大家照样得开车上班；某个奶茶品牌涨 10%，很多人立刻换别家。这种“敏感程度”的差别，就是 price elasticity of demand（PED）。它决定了一件很实际的事：涨价以后，商家的收入是增加还是减少？

定义与公式 DEFINITION PED = % change in quantity demanded% change in price PED measures how responsive quantity demanded is to a change in the good's own price. 因为价格和需求量反向变化，算出来总是负数，AP 习惯取绝对值来判断大小（写负号也算对）。

The Midpoint Method 中点法

普通的百分比用“原来的值”做分母，从 A 到 B 和从 B 到 A 会算出不同的结果。中点法用两个值的平均数做分母，正反方向结果相同：

公式 FORMULA %ΔQ = Q₂ − Q₁(Q₁ + Q₂) / 2　　%ΔP = P₂ − P₁(P₁ + P₂) / 2

WORKED EXAMPLE 3奶茶涨价 $1，卖得少了多少？原创

A bubble tea shop raises its price from $5 to $6 per cup, and daily sales fall from 100 cups to 80 cups. (a) Calculate the price elasticity of demand using the midpoint method. (b) Is demand elastic or inelastic? (c) What happens to the shop's total revenue?

(a) %ΔQ = (80 − 100) ÷ 90 = −22.2%；%ΔP = (6 − 5) ÷ 5.5 = 18.2%；PED = −22.2% ÷ 18.2% ≈ −1.22（绝对值 1.22）。

(b) |PED| = 1.22 > 1 → elastic：需求量变化的幅度大于价格变化的幅度。

(c) TR 从 5 × 100 = $500 降到 6 × 80 = $480，下降。需求富有弹性时，涨价会让总收益下降。

对比：如果用普通百分比，$5 → $6 得 20% ÷ 20% = 1.0，$6 → $5 却得 25% ÷ 16.7% = 1.5，方向不同结果不同。AP 评分对两种算法都接受，但同一题内要一致。

Five Ranges of PED 五种情况

Perfectly inelastic 完全无弹性　|PED| = 0　垂直线
价格怎么变，需求量都不变。例：胰岛素对糖尿病患者、救命药。

Inelastic 缺乏弹性　0 < |PED| < 1
需求量变化的百分比 < 价格变化的百分比。例：汽油、食盐、香烟、学校的必修教材。

Unit elastic 单位弹性　|PED| = 1
两个百分比正好相等，价格变动时总收益不变。

Elastic 富有弹性　|PED| > 1
需求量变化的百分比 > 价格变化的百分比。例：某一个奶茶品牌、电影票、海外旅游。

Perfectly elastic 完全弹性　|PED| = ∞　水平线
价格稍高一点，需求量就降到 0。例：完全竞争市场里单个农民的小麦（Unit 3）。

技巧 · 一句话判断看“谁变得多”：数量变化百分比 > 价格变化百分比 → elastic；数量变化百分比 < 价格变化百分比 → inelastic。画图时，在同一张图上比较两条经过同一点的曲线，越陡越缺乏弹性；但不要用斜率去比较同一条直线上的不同点。

Determinants of PED 影响需求弹性的因素（CED MKT-3.E.4）

🔄

替代品多不多（最重要）：替代品越多越容易换 → 越有弹性。某个奶茶品牌（替代品很多）比“饮料”这个大类有弹性得多。

💊

必需品还是奢侈品：必需品（药、水电、食盐）缺乏弹性；可有可无的东西（演唱会、名牌包）富有弹性。

🧾

占收入的比例：一包盐涨价 20% 你几乎感觉不到；一辆车涨价 20% 就会让很多人推迟购买 → 占比越大越有弹性。

⏳

时间长短：油价刚涨时只能照常开车（短期缺乏弹性）；时间一长，可以换电动车、搬到公司附近 → 长期更有弹性。

🏷️

市场定义的宽窄：“食物”几乎无弹性，“汉堡”有一定弹性，“某家店的汉堡”非常有弹性。

Elasticity Along a Linear Demand Curve & Total Revenue 线性需求曲线与总收益

Price P	$10	$9	$8	$7	$6	$5	$4	$3	$2	$1	$0
Quantity Q	0	1	2	3	4	5	6	7	8	9	10
TR = P × Q	0	9	16	21	24	25	24	21	16	9	0

一条直线的需求曲线，斜率处处相同，但弹性一路在变：中点以上 elastic，中点 unit elastic，中点以下 inelastic。Slope is not elasticity.（CED MKT-3.E.2）

同一条需求曲线 P = 10 − Q 对应的总收益：TR 在中点（Q = 5, P = $5）达到最大 $25。

Total Revenue Test 总收益检验（CED MKT-3.E.5）

	Price ↑	Price ↓
Elastic \|PED\| > 1	TR ↓	TR ↑
Inelastic \|PED\| < 1	TR ↑	TR ↓
Unit elastic \|PED\| = 1	TR 不变	TR 不变

记法：inelastic → 价格和 TR 同方向；elastic → 价格和 TR 反方向。反过来，看到“降价后 TR 增加”就能推出需求在这一段 elastic。消费者的总支出（total expenditure）就是卖家的总收益，规律相同。

WORKED EXAMPLE 4同一条直线，弹性为什么不同？原创

Using the demand curve P = 10 − Q, calculate the midpoint price elasticity of demand (a) when the price falls from $8 to $7, and (b) when the price falls from $3 to $2. Explain why the answers differ even though the slope is the same.

(a) Q：2 → 3。%ΔQ = 1 ÷ 2.5 = 40%，%ΔP = 1 ÷ 7.5 = 13.3% → |PED| = 3.0，elastic；TR 从 16 增加到 21。

(b) Q：7 → 8。%ΔQ = 1 ÷ 7.5 = 13.3%，%ΔP = 1 ÷ 2.5 = 40% → |PED| = 0.33，inelastic；TR 从 21 减少到 16。

原因：两段的价格和数量都是变化 1 个单位（斜率相同），但弹性看的是百分比。高价区数量很少，多 1 个就是很大的百分比；价格很高，降 $1 只是很小的百分比，所以高价区弹性大。

WORKED EXAMPLE 5谷贱伤农：丰收了，农民为什么反而发愁？原创

After a record harvest, the price of apples falls from $50 to $30 per box, while the quantity sold rises from 1,000 to 1,200 boxes. (a) Calculate the price elasticity of demand. (b) What happens to apple growers' total revenue? Explain using elasticity.

丰收 → 供给右移。因为粮食的需求缺乏弹性，价格大跌（P₁ → P₂）而销量只多一点（Q₁ → Q₂），农民的总收入反而下降。

(a) %ΔQ = 200 ÷ 1,100 = 18.2%，%ΔP = −20 ÷ 40 = −50% → |PED| ≈ 0.36，inelastic。

(b) TR 从 50 × 1,000 = $50,000 降到 30 × 1,200 = $36,000。需求缺乏弹性，价格下降的百分比大于销量增加的百分比，所以总收益下降。这也是很多国家对农产品实行价格支持的原因之一（见 2.8 price floor）。

LAB 2弹性与总收益 · Elasticity & Total Revenue

需求曲线 P = 10 − Q。拖动滑块改变价格：蓝色矩形的面积 = 总收益 TR = P × Q；下方显示从这个价格降价 $0.5 时的中点弹性和 TR 变化。

价格 P = $7

中点法计算器 Midpoint Calculator

P₁ Q₁ 　→　P₂ Q₂

类型：

练习 Practice 2.3

以下为历年 AP 真题，点选项即时判分并显示解析。

2.3-12005 AP Micro Q3

To alleviate a financial crisis, a university increases student fees. This action will increase university revenues if the price elasticity of demand for university education is

inelastic
unit elastic
elastic
equal to the price elasticity of supply
equal to one

需求缺乏弹性时，价格与总收益同方向变化：涨价 → 收入增加。

2.3-22012 AP Micro Q34

Promoters of a rock group know that if they charged $8 a ticket, 400 people would buy tickets for a concert, and if they charged $4 a ticket, 800 people would buy tickets. Over this price range, the demand for the concert tickets for the rock group is

elastic
inelastic
unit elastic
perfectly elastic
perfectly inelastic

中点法：%ΔQ = 400 ÷ 600 = 66.7%，%ΔP = 4 ÷ 6 = 66.7%，PED = 1 → unit elastic。也可以用总收益判断：$8 × 400 = $4 × 800 = $3,200，TR 不变。

2.3-32013 AP Micro Q35

Moving from left to right along a downward-sloping linear demand curve, price elasticity varies in which of the following ways?

First unit elastic, then inelastic throughout
First unit elastic, then elastic throughout
First inelastic, then unit elastic throughout
First elastic, then unit elastic, and finally inelastic
First inelastic, then unit elastic, and finally elastic

线性需求曲线从左上到右下：中点以上 elastic，中点 unit elastic，中点以下 inelastic。

2.3-42014 AP Micro 练习卷 Q12

Which of the following will tend to make the demand for a product more elastic?

New firms which produce similar products enter the industry.
A change in taste and preferences makes the product more desirable.
The product is necessary for use with a complement.
Production of the product is protected by a patent.
Production cost of the product decreases.

同类替代品越多，消费者越容易换，需求越有弹性。(D) 专利让替代品更少，需求更缺乏弹性。

2.3-52014 AP Micro 练习卷 Q18

A city transit authority increases the price of subway and bus tickets from $1.25 to $1.50. If the demand for these tickets is price inelastic, the number of people riding buses and subways and the city’s revenues will most likely change in which of the following ways?

Number of People Riding: Increase；City’s Revenues: Increase
Number of People Riding: Decrease；City’s Revenues: Increase
Number of People Riding: Decrease；City’s Revenues: Decrease
Number of People Riding: Decrease；City’s Revenues: Remain constant
Number of People Riding: Remain constant；City’s Revenues: Increase

需求缺乏弹性：涨价后乘客会少一些（需求量下降，但下降的百分比小于涨价的百分比），总收入增加。

2.3-62016 AP Micro 练习卷 Q33

If a 10 percent increase in the price of a good leads to a 25 percent decrease in the quantity demanded of the good, demand is

relatively inelastic
relatively elastic
unit elastic
perfectly elastic
perfectly inelastic

|PED| = 25% ÷ 10% = 2.5 > 1 → 相对富有弹性。

2.3-72019 AP Micro 练习卷 Q5

Which of the following relationships among the price elasticity of demand, change in price, and change in total revenue is consistent?

Price Elasticity of Demand: Elastic；Change in Price: Increase；Change in Total Revenue: Increase
Price Elasticity of Demand: Elastic；Change in Price: Decrease；Change in Total Revenue: Decrease
Price Elasticity of Demand: Unit Elastic；Change in Price: Decrease；Change in Total Revenue: Decrease
Price Elasticity of Demand: Inelastic；Change in Price: Decrease；Change in Total Revenue: Increase
Price Elasticity of Demand: Inelastic；Change in Price: Decrease；Change in Total Revenue: Decrease

缺乏弹性时价格与 TR 同方向：降价 → TR 下降 ✓。(A) elastic 涨价 TR 应下降；(B) elastic 降价 TR 应上升；(C) unit elastic TR 不变；(D) inelastic 降价 TR 应下降。

2.3-82022 AP Micro 国际卷 Q46

The postmaster general of a country decides to double the price of first-class stamps, expecting the action to cause the revenues from sales of first-class stamps to double. What absolute value of price elasticity of demand is the postmaster general assuming in making the projection?

0
Between 0 and 1
1
Greater than 1
Infinite

价格翻倍而收入也翻倍，说明销量完全没变 → 需求完全无弹性，|PED| = 0。

2.3-92025 AP Micro 国际卷 Q23

Assume unusually cold weather destroys some of a nation’s strawberry crop, which results in an increase in total revenue for strawberry producers. Which of the following must be true?

The market supply of strawberries is elastic.
The market supply of strawberries is inelastic.
The market demand for strawberries is elastic.
The market demand for strawberries is unit elastic.
The market demand for strawberries is inelastic.

供给减少使价格上升，而总收益反而增加 → 价格上涨的百分比大于数量下降的百分比 → 需求缺乏弹性。

2.4Price Elasticity of Supply 供给的价格弹性

价格上涨时，卖家能多快、多大程度地增加供给？今晚演唱会的座位就那么多，票价涨到天上也变不出新座位；手机 App 的“供给”几乎可以无限复制。

定义与公式 DEFINITION PES = % change in quantity supplied% change in price 供给定律下价格和供给量同方向变化，所以 PES 是正数。判断标准同样以 1 为界：PES > 1 elastic，< 1 inelastic，= 1 unit elastic（CED MKT-3.E.7）。

Perfectly inelastic supply　PES = 0　垂直线：数量完全固定。例：梵高的真迹、今晚体育馆的座位、某块土地。

Perfectly elastic supply　PES = ∞　水平线：在某个价格下想要多少都能供给。例：可以无限复制的数字产品、长期中的某些标准化商品。

Determinants of PES 影响供给弹性的因素

⏳

时间（最重要）：短期内来不及扩建工厂、种新的果树 → 缺乏弹性；长期可以调整一切 → 更有弹性。

🔧

投入品是否容易获得、能否在不同用途间转换：生产 T 恤的布料、工人随处可得 → 有弹性；稀有矿产、需要多年培训的医生 → 缺乏弹性。CED 特别提到：替代性投入品的价格（price of alternative inputs）会影响 PES。

🏭

是否有闲置产能：工厂还有空闲机器 → 很快就能多生产 → 有弹性。

📦

能否储存、有没有库存：有库存的商品涨价时能马上多卖；新鲜草莓不能存 → 缺乏弹性。

价格从 A 点上涨：短期内工厂、土地来不及调整，供给量只能增加一点（陡）；时间越长，企业可以扩建、新企业可以进入，供给量增加得更多（平）。

WORKED EXAMPLE 6牛油果涨价了，果园能多供多少？原创

When the price of avocados rises from $2.00 to $2.50 per pound, the quantity supplied by local growers rises from 100 tons to 150 tons. Calculate the price elasticity of supply using the midpoint method and classify it.

%ΔQs = 50 ÷ 125 = 40%；%ΔP = 0.5 ÷ 2.25 = 22.2%；PES = 40% ÷ 22.2% ≈ 1.8 → elastic。

如果用普通百分比：50% ÷ 25% = 2.0，也是 elastic。真题 2016 FRQ Q1 就考了用这种方法算 PES（见本讲义最后的 FRQ 1）。

练习 Practice 2.4

以下为历年 AP 真题，点选项即时判分并显示解析。

2.4-12014 AP Micro 练习卷 Q2

The quantity of peanuts supplied increased from 40 tons per week to 60 tons per week when the price of peanuts increased from $4 per ton to $5 per ton. The price elasticity of supply for peanuts over this price range is

elastic
inelastic
unit elastic
perfectly elastic
perfectly inelastic

中点法：%ΔQ = 20 ÷ 50 = 40%，%ΔP = 1 ÷ 4.5 = 22.2% → PES ≈ 1.8 > 1，elastic。（普通百分比：50% ÷ 25% = 2，结论相同。）

2.4-22019 AP Micro 练习卷 Q32

If the price elasticity of supply for pickles is 2 and the price of pickles increases by 10 percent, then the quantity supplied of pickles will increase by

0.2%
5%
8%
12%
20%

%ΔQs = PES × %ΔP = 2 × 10% = 20%。

2.4-32021 AP Micro 国际卷 Q58

A market supply curve is more elastic in the long run than in the short run because

producers’ marginal costs are increasing
the quantity demanded increases at a steady rate
there are no fixed inputs in the long run
of constant returns to scale in production
of diseconomies of scale in production

长期中所有投入都可以调整（没有固定投入），企业可以扩建、新企业可以进入，所以供给更有弹性。

2.4-42023 AP Micro 国际卷 Q51

If the value of the price elasticity of supply for a product is 0.5, which of the following statements is correct?

A 10 percent increase in the price of the product will decrease the quantity supplied by 5 percent.
A 10 percent increase in the price of the product will decrease the quantity supplied by 20 percent.
A 10 percent increase in the price of the product will increase the quantity supplied by 20 percent.
A 20 percent decrease in the price of the product will decrease the quantity supplied by 10 percent.
A 20 percent decrease in the price of the product will decrease the quantity supplied by 40 percent.

PES = 0.5：价格变化 20%，供给量同方向变化 0.5 × 20% = 10%。(A)(B) 方向错（供给量与价格同方向）；(C)(E) 数值错。

2.4-52025 AP Micro 国际卷 Q9

Price ($)	Quantity supplied
12	60
14	75
16	90
18	120

The table shows the market supply schedule for a good.

When the price changes from $14 to $16, the supply in this range is

elastic
inelastic
unit elastic
perfectly elastic
perfectly inelastic

$14 → $16：中点法 %ΔQ = 15 ÷ 82.5 = 18.2%，%ΔP = 2 ÷ 15 = 13.3% → PES ≈ 1.36 > 1，elastic。

2.5Other Elasticities 其他弹性：收入弹性与交叉弹性

弹性可以衡量需求对任何决定因素的反应（CED MKT-3.E.9）。最常考的另外两种：需求对收入的反应、对另一种商品价格的反应。这两种弹性的正负号本身就有意义，不能取绝对值。

Income Elasticity of Demand 需求的收入弹性 (YED) YED = % change in quantity demanded% change in income

YED	商品类型	例子
> 0	Normal good 正常品（收入↑ 买更多）	大多数商品
> 1	其中 luxury 奢侈品（收入弹性大）	海外旅游、名牌包、私教课
0 ~ 1	其中 necessity 必需品	大米、水电、牙膏
< 0	Inferior good 劣等品（收入↑ 买更少）	公交车、二手衣服、最便宜的方便面

Cross-Price Elasticity of Demand 需求的交叉价格弹性 (XED) XED_A,B = % change in quantity demanded of good A% change in price of good B

XED	关系	例子
> 0	Substitutes 替代品：B 涨价 → 人们改买 A	可口可乐与百事、地铁与网约车
< 0	Complements 互补品：B 涨价 → A 也少买	游戏机与游戏、打印机与墨盒
≈ 0	Unrelated 无关	牙膏与演唱会门票

数值的绝对值越大，两种商品的关系越密切。

WORKED EXAMPLE 7算一算，再判断商品类型原创

(a) When average income rises by 10%, the quantity of overseas trips demanded rises by 25%, the quantity of bus rides demanded falls by 5%, and the quantity of rice demanded rises by 3%. Calculate each YED and classify each good.
(b) When the price of coffee rises by 20%, the quantity of tea demanded rises by 10%. When the price of printers rises by 10%, the quantity of ink cartridges demanded falls by 15%. Calculate each XED and identify the relationship.

(a) 海外旅游 25% ÷ 10% = 2.5 → 正常品中的奢侈品；公交 −5% ÷ 10% = −0.5 → 劣等品；大米 3% ÷ 10% = 0.3 → 正常品中的必需品。

(b) 茶与咖啡：10% ÷ 20% = +0.5 → 替代品；墨盒与打印机：−15% ÷ 10% = −1.5 → 互补品。

用弹性反推数量：如果 XED = −2，B 涨价 10%，A 的需求量就下降 2 × 10% = 20%（真题 2016 FRQ Q1(c)(iii) 的考法：100 杯 → 80 杯）。

总结 · 四种弹性一张表

弹性	分子：%Δ…	分母：%Δ…	看什么
PED	Qd of the good	its own price	\|PED\| 与 1 比大小（负号可忽略）
PES	Qs of the good	its own price	与 1 比大小（正数）
YED	Qd of the good	income	正负号 → 正常品/劣等品；>1 → 奢侈品
XED	Qd of good A	price of good B	正负号 → 替代品/互补品

LAB 3看数字分类 · Classify by Elasticity

根据给出的弹性数值判断商品类型。（得分：0 / ）

练习 Practice 2.5

以下为历年 AP 真题，点选项即时判分并显示解析。

2.5-12010 AP Micro Q6

Following a decrease in the supply of oranges, the price of orange juice increased by 20 percent, which resulted in a 10 percent increase in the quantity of apple juice consumed. This implies that the cross elasticity of demand between orange juice and apple juice is

0
0.5
1
2
indeterminate

XED = 10% ÷ 20% = 0.5，为正 → 橙汁与苹果汁是替代品。

2.5-22017 AP Micro 练习卷 Q33

Carmen consumes both entertainment and medical care. For Carmen, entertainment is a normal good, and the income elasticity of her demand for medical care is zero. If Carmen’s income increases, which of the following will be the immediate impact on her consumption?

Entertainment: Increase；Medical Care: Increase
Entertainment: Increase；Medical Care: No change
Entertainment: No change；Medical Care: Increase
Entertainment: Increase；Medical Care: Decrease
Entertainment: Decrease；Medical Care: Increase

娱乐是正常品 → 收入增加时消费增加；医疗的收入弹性为 0 → 不变。

2.5-32019 AP Micro 练习卷 Q33

If a 10 percent increase in the price of good X results in a 20 percent decrease in the quantity of good Y demanded, which of the following is true?

Good X and good Y are complementary goods, and the cross-price elasticity is −0.5.
Good X and good Y are substitute goods, and the income elasticity is +2.
Good X and good Y are complementary goods, and the cross-price elasticity is −2.
Good X and good Y are normal goods, and the income elasticity is +2.
Good X and good Y are substitute goods, and the cross-price elasticity is −2.

XED = −20% ÷ 10% = −2，为负 → 互补品。

2.5-42021 AP Micro 国际卷 Q26

A 10 percent increase in Sandra’s income causes Sandra’s consumption of milk to decrease from 10 cartons to 6 cartons. Sandra’s income elasticity of demand for milk is

greater than zero and therefore milk is a necessity
greater than zero and therefore milk is a normal good
less than zero and therefore milk is a normal good
less than zero and therefore milk is an inferior good
greater than one and therefore milk is a necessity

收入增加而牛奶消费减少 → 收入弹性为负 → 劣等品。

2.5-52022 AP Micro 国际卷 Q18

When Tom’s income increases from $100 to $120, the quantity of golf balls he purchases increases from 10 to 12. What is Tom’s income elasticity of demand, and what type of good is a golf ball for Tom?

Income elasticity of demand is 1, and golf balls are a normal good for Tom.
Income elasticity of demand is −1, and golf balls are an inferior good for Tom.
Income elasticity of demand is 10, and golf balls are a normal good for Tom.
Income elasticity of demand is −10, and golf balls are an inferior good for Tom.
Income elasticity of demand is 20, and golf balls are a normal good for Tom.

YED = (2/10) ÷ (20/100) = 20% ÷ 20% = 1 > 0 → 正常品。

2.5-62023 AP Micro 国际卷 Q25

Cross-price elasticity of demand for honey with respect to yogurt is −4. At the current equilibrium, consumers are buying 100 containers of yogurt per week and 20 containers of honey per week. If the price of yogurt increases by 10 percent per container, what will be the new quantity demanded of honey?

4 containers
8 containers
10 containers
12 containers
40 containers

%ΔQ(蜂蜜) = −4 × 10% = −40%，20 × (1 − 0.4) = 12。

2.5-72025 AP Micro 国际卷 Q19

A 5% increase in the price of Good A results in a 10% decrease in the quantity demanded of Good B. What is the cross-price elasticity between Good A and Good B, and what does this indicate about the relationship between these two goods?

Cross-Price Elasticity: −1；Relationship: Complements
Cross-Price Elasticity: −2；Relationship: Complements
Cross-Price Elasticity: 0.5；Relationship: Complements
Cross-Price Elasticity: −2；Relationship: Substitutes
Cross-Price Elasticity: 0.5；Relationship: Substitutes

XED = −10% ÷ 5% = −2，为负 → 互补品。

2.6Market Equilibrium & Consumer/Producer Surplus 市场均衡与剩余

把需求曲线和供给曲线画在同一张图上，交点就是市场的均衡（equilibrium）。这个点不是谁规定的，而是买卖双方自愿交易的结果。

Price	Qd	Qs	结果
$2	10	2	短缺 8
$4	8	4	短缺 4
$6	6	6	均衡
$8	4	8	过剩 4
$10	2	10	过剩 8

均衡点 E：P = $6 时，买家想买 6 千杯，卖家也正好想卖 6 千杯，市场出清（market clears）。

定义 DEFINITION Market equilibrium: the price at which quantity demanded equals quantity supplied (Qd = Qs). There is no shortage or surplus, so there is no pressure for the price to change.
均衡价格本身是一种信号：它告诉生产者生产多少、告诉消费者买多少，引导资源配置（CED MKT-4.A.3）。

Consumer Surplus 消费者剩余

你心里愿意为一张演唱会门票最多付 $100，结果只花 $50 就买到了，多出来的 $50 就是你的“赚到”。

定义 DEFINITION Consumer surplus (CS) = willingness to pay − price actually paid, summed over all buyers. 图上：需求曲线以下、价格线以上的面积。

5 位歌迷的最高愿付价格（WTP）排成“台阶”。票价 $50 时，A、B、C 买票，消费者剩余 = 50 + 30 + 10 = $90。

Producer Surplus 生产者剩余

定义 DEFINITION Producer surplus (PS) = price received − the lowest price the seller is willing to accept (the seller's cost), summed over all sellers. 图上：价格线以下、供给曲线以上的面积。

5 位学长卖二手教材，每人愿意接受的最低价（成本）也排成台阶。价格 $35 时，V、W、X 卖出，生产者剩余 = 25 + 15 + 5 = $45。

Surplus in a Market Graph 市场图中的剩余

需求曲线以下、价格以上 = 消费者剩余（蓝）；价格以下、供给曲线以上 = 生产者剩余（绿）。两者之和 = 总剩余（total economic surplus）。

WORKED EXAMPLE 8算出剩余原创

In the bubble tea market above (quantity in thousands of cups), demand is P = 12 − Q and supply is P = Q. Calculate consumer surplus, producer surplus, and total economic surplus at equilibrium.

均衡：12 − Q = Q → Q = 6，P = $6。

CS = ½ × 底 × 高 = ½ × 6 × (12 − 6) = 18（千美元）。

PS = ½ × 6 × (6 − 0) = 18（千美元）。

总剩余 = 18 + 18 = 36（千美元）。三角形的“高”一定是价格差（纵轴），“底”是数量（横轴）。

如果只生产 4 单位，第 5、6 单位本来 MB > MC、能创造剩余却没有被生产，红色三角形的剩余就丢了。只有在均衡数量处，总剩余最大——竞争市场是有效率的（CED MKT-4.A.5）。

LAB 4市场均衡实验室 · Market Lab

基准市场：需求 P = 12 − Q，供给 P = Q（均衡 P = $6, Q = 6）。移动两条曲线，看价格、数量和剩余怎么变；也可以把价格固定在某个水平，看短缺或过剩。

需求移动：0

供给移动：0

显示 CS / PS 把价格固定在 $4

练习 Practice 2.6

以下为历年 AP 真题，点选项即时判分并显示解析。

2.6-12005 AP Micro Q32

Consumer surplus is defined as

opportunity cost minus total revenue
total revenue minus opportunity cost
the difference between the resource costs and the price that consumers pay
the difference between the value that consumers place on a good and the price they pay
the sum of external costs and benefits

消费者剩余 = 消费者愿意支付的价值（WTP）与实际支付价格之差。

2.6-22013 AP Micro Q3

按原题重绘

The diagram above shows the demand curve for a good. If the price increases from P₁ to P₂, and quantity consumed decreases from Q₂ to Q₁, consumer surplus decreases by the area

BDC
P1P2BC
P1P2BD
Q1DCQ2
P2P3B

价格从 P₁ 涨到 P₂：原来的 CS 是 P₃CP₁ 三角形，现在是 P₃BP₂，减少的部分是梯形 P₁P₂BC（既包括继续购买的单位多付的钱，也包括不再购买的单位损失的剩余）。

2.6-32013 AP Micro Q6

Price	Jill's demand	All other consumers' demand	Market supply
$8	140	5,200	5,750
$7	150	5,400	5,550
$6	160	5,600	5,350
$5	170	5,800	5,150
$4	180	6,000	4,950

The table above shows the demand schedules for Jill and for all other consumers, as well as the market supply schedule for the same good. The equilibrium price in this market is

市场需求 = Jill + 其他人。$7 时：150 + 5,400 = 5,550 = 供给 5,550 → 均衡价格 $7。$8 时需求 5,340 < 供给 5,750（过剩），$6 时需求 5,760 > 供给 5,350（短缺）。

2.6-42016 AP Micro 练习卷 Q20

A perfectly competitive market in equilibrium is allocatively efficient and it maximizes

total consumer surplus
total producer surplus
the sum of total consumer surplus and total producer surplus
total revenue
total external benefits

完全竞争的均衡使总剩余（CS + PS）最大，这就是配置效率。

2.6-52017 AP Micro 练习卷 Q10

按原题重绘

Given the demand curve above, if price decreases from $8 to $4 per unit, total consumer surplus will

increase by $4
increase by $8
increase by $12
increase by $18
remain constant at $24

需求曲线纵截距 12。P = $8 时 CS = ½ × 3 × (12 − 8) = 6；P = $4 时 CS = ½ × 6 × (12 − 4) = 24。增加 24 − 6 = $18。

2.6-62017 AP Micro 练习卷 Q42

If a perfectly competitive market with no government intervention is allocatively efficient, which of the following must be true?

Consumer surplus plus producer surplus is at its maximum.
Consumer surplus is larger than producer surplus.
Producer surplus is larger than consumer surplus.
All surplus is producer surplus.
All surplus is consumer surplus.

配置效率 = 总剩余（CS + PS）最大。CS 与 PS 的相对大小取决于曲线形状，不一定谁大。

2.6-72025 AP Micro 国际卷 Q6

Kareem is willing to pay $25 for a new remote control produced by Remotek in a perfectly competitive market. Which of the following is true if the market price of the remote control is $10?

Kareem’s consumer surplus is $10.
Kareem’s consumer surplus is $15.
Kareem’s consumer surplus is $25.
Remotek’s producer surplus is $25.
Remotek’s producer surplus is $35.

消费者剩余 = WTP − 价格 = 25 − 10 = $15。题目没有给出生产成本，无法算生产者剩余。

2.7Disequilibrium & Changes in Equilibrium 失衡与均衡的变动

Shortage & Surplus 短缺与过剩

价格高于均衡 → 卖不完（surplus）→ 卖家降价；价格低于均衡 → 不够卖（shortage）→ 买家抢购、价格被抬高。市场力量把价格推回均衡（CED MKT-4.B.1）。

🚄

春运火车票：票价不随需求上涨，春节前 Qd 远大于 Qs → 长期短缺 → 熬夜抢票、黄牛出现。

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水果大量上市：价格定高了卖不完 → 过剩 → 摊主傍晚降价清货，价格回落到均衡。

易错 TRAP 短缺（shortage）发生在价格低于均衡时，过剩（surplus）发生在价格高于均衡时。别和 1.1 的“稀缺（scarcity）”搞混，也别和 2.6 的“消费者剩余（consumer surplus）”搞混——英文都叫 surplus，含义完全不同。

One Curve Shifts 单条曲线移动

Demand ↑（D 右移）　P↑　Q↑
例：天气变热，冰淇淋需求增加。PS 一定增加。

Demand ↓（D 左移）　P↓　Q↓
例：健康新闻说含糖饮料有害。PS 一定减少。

Supply ↑（S 右移）　P↓　Q↑
例：新技术让芯片成本下降。CS 一定增加。

Supply ↓（S 左移）　P↑　Q↓
例：霜冻毁掉橙子。CS 一定减少。

四步法 FOUR STEPS ① 画出原来的均衡 ② 判断事件影响的是 D 还是 S（还是都影响）③ 判断方向，画出新曲线 ④ 比较新旧均衡的 P、Q。FRQ 里一定要标出 P₁、Q₁ 和 P₂、Q₂。

考纲细节 · 变化有多大，取决于弹性（CED MKT-4.B.2）同样的冲击，价格和数量变多少，要看另一条曲线有多陡。

需求缺乏弹性：供给减少后，价格涨很多、数量只少一点。

需求富有弹性：同样的供给减少，价格只涨一点，数量却少很多。

Both Curves Shift 双移动

两条曲线同时移动时，P 和 Q 中一定有一个不确定（indeterminate），因为两个移动对它的影响方向相反，结果取决于谁移得多。

Demand	Supply	Price	Quantity
↑	↑	?	↑
↓	↓	?	↓
↑	↓	↑	?
↓	↑	↓	?

技巧 · 分开画，再合起来先只移 D，写下 P、Q 的方向；再只移 S，写下方向。两次方向相同的那个变量是确定的，方向相反的那个就是“?”。不要靠一张图上随手画的大小来下结论——除非题目告诉你哪个移动更大。

需求增加 + 供给减少：价格一定上涨；数量取决于哪条曲线移动得多（这张图里需求移动得稍多，数量略增）。

WORKED EXAMPLE 9春节前的饺子市场原创

Before the Spring Festival, families want to buy more frozen dumplings. At the same time, many factory workers go home for the holiday, reducing the output of dumpling factories. What happens to the equilibrium price and quantity of frozen dumplings? What would you need to know to determine the change in quantity?

偏好/节日需求 → D 右移：P↑，Q↑。工人回家 → S 左移：P↑，Q↓。

价格两次都上涨 → P 一定上涨；数量一增一减 → Q 不确定。

要判断数量，需要知道两条曲线移动的相对大小：需求增加得更多 → Q 上升；供给减少得更多 → Q 下降。

一个市场的变化会通过“替代品、互补品、投入品”传到别的市场。这正是真题长 FRQ 最爱的考法（例如 2016 FRQ Q1：香蕉 → 松饼 → 咖啡）。

WORKED EXAMPLE 10一场猪瘟引发的连锁反应原创

An outbreak of African swine fever kills a large share of pigs. Explain the effect on (a) the market for pork, (b) the market for beef (a substitute for pork), (c) the market for pork dumplings (pork is an input), and (d) the market for soy-based meat alternatives if pork and soy meat are substitutes.

(a) 猪肉：生猪减少 → 猪肉 S 左移 → P↑，Q↓。

(b) 牛肉：替代品（猪肉）涨价 → 牛肉 D 右移 → 牛肉 P↑，Q↑。

(c) 猪肉饺子：投入品（猪肉）涨价 → 生产成本↑ → 饺子 S 左移 → P↑，Q↓。

(d) 植物肉：同 (b)，替代品涨价 → D 右移 → P↑，Q↑。

关键：每一步先问“这个变化在这个市场里影响的是 D 还是 S”。投入品价格影响的是供给，替代品/互补品价格影响的是需求。

练习 Practice 2.7

以下为历年 AP 真题，点选项即时判分并显示解析。

2.7-12000 AP Micro Q3

In a perfectly competitive market, which of the following shifts in the supply and demand curves will definitely cause both the equilibrium price and quantity to decrease?

Supply Curve: Shifts to the left；Demand Curve: Shifts to the left
Supply Curve: Shifts to the left；Demand Curve: No shift
Supply Curve: Shifts to the right；Demand Curve: Shifts to the right
Supply Curve: No shift；Demand Curve: Shifts to the right
Supply Curve: No shift；Demand Curve: Shifts to the left

只有需求左移（供给不变）才一定使 P 和 Q 都下降。(A) 两条曲线都左移时价格不确定；(B) 供给左移使价格上升。

2.7-22005 AP Micro Q19

Assume that consumers consider potatoes to be an inferior good, but consider rice to be a normal good. An increase in consumers' incomes will most likely affect the equilibrium price and quantity of potatoes and rice in which of the following ways?

Potatoes Price: Decrease；Potatoes Quantity: Decrease；Rice Price: Increase；Rice Quantity: Increase
Potatoes Price: Decrease；Potatoes Quantity: Increase；Rice Price: Increase；Rice Quantity: Decrease
Potatoes Price: Decrease；Potatoes Quantity: Decrease；Rice Price: Decrease；Rice Quantity: Decrease
Potatoes Price: Increase；Potatoes Quantity: Increase；Rice Price: Decrease；Rice Quantity: Decrease
Potatoes Price: Increase；Potatoes Quantity: Decrease；Rice Price: Increase；Rice Quantity: Decrease

土豆是劣等品 → 收入增加使需求左移 → P↓ Q↓；大米是正常品 → 需求右移 → P↑ Q↑。

2.7-32005 AP Micro Q50

Leather and beef are jointly produced such that an increase in the production of one results in an equal increase in the production of the other. An increase in the demand for leather will most likely cause

a decrease in the price of leather
a decrease in the price of beef
a decrease in the equilibrium quantity of beef sold
an increase in the demand for beef in the short run
an increase in the supply of leather

皮革与牛肉是联合产品（joint supply）。皮革需求增加 → 宰杀更多牛 → 牛肉供给增加 → 牛肉价格下降。

2.7-42012 AP Micro Q3

Assume that consumers consider popcorn and pretzels to be substitutes. A significant decrease in the supply of popcorn will affect the pretzel market by

increasing the demand for pretzels and therefore the supply of pretzels
increasing the demand for pretzels and therefore the price of pretzels
decreasing the demand for pretzels and therefore the price of pretzels
increasing the supply of pretzels and therefore the price of pretzels
decreasing the supply of pretzels and therefore the price of pretzels

爆米花供给减少 → 爆米花涨价 → 替代品椒盐卷饼的需求增加 → 椒盐卷饼价格上涨。

2.7-52014 AP Micro 练习卷 Q48

Given an increase in the price of material K—which is an input used to produce good X—and an increase in the price of good Y—which is a substitute for good X—which of the following will definitely occur?

The equilibrium price of good X will decrease.
The equilibrium price of good X will increase.
The equilibrium quantity of good X will be unaffected.
The equilibrium quantity of good X will increase.
The equilibrium quantity of good X will decrease.

投入品 K 涨价 → X 供给左移（P↑ Q↓）；替代品 Y 涨价 → X 需求右移（P↑ Q↑）。价格两次都上升 → 一定上升；数量不确定。

2.7-62015 AP Micro 练习卷 Q4

Last year 17 million tons of beans were sold for $300 per ton. This year 17 million tons of beans were sold for $285 per ton. Which of the following changes in demand and supply could have caused this outcome?

Demand: Increase；Supply: Increase
Demand: Increase；Supply: Decrease
Demand: Decrease；Supply: Increase
Demand: Decrease；Supply: Decrease
Demand: Increase；Supply: No change

数量不变、价格下降：需求减少（P↓ Q↓）与供给增加（P↓ Q↑）同时发生，数量的变化正好抵消。

2.7-72015 AP Micro 练习卷 Q47

If shirts and ties are complements and if the price of shirts increases due to an increase in the price of cotton, which of the following is most likely to occur in the market for ties in the short run?

The equilibrium price and quantity of ties will increase.
The equilibrium price and quantity of ties will decrease.
The equilibrium price of ties will increase and the equilibrium quantity will decrease.
The supply of ties will increase.
The demand for ties will increase.

棉花涨价 → 衬衫涨价 → 衬衫的互补品领带需求减少 → 领带的价格和数量都下降。

2.7-82018 AP Micro 练习卷 Q48

Assume that more corn is used to produce ethanol. Simultaneously, more effective control of pests and weeds occurs in farming. Which of the following will definitely occur in the corn market?

The price of corn will increase.
The quantity of corn will decrease.
The price of corn will decrease.
The quantity of corn will increase.
The supply of corn will decrease.

更多玉米用于生产乙醇 → 玉米需求增加；病虫害控制更好 → 供给增加。两者都让数量增加 → 数量一定增加，价格不确定。

2.7-92019 AP Micro 练习卷 Q3

An increase in the supply of good X resulted in an increase in the price and quantity of good Y. It can be concluded that good Y is

an inferior good
a luxury good
a normal good
a substitute for good X
a complement for good X

X 供给增加 → X 降价。Y 的价格和数量都上升，说明 Y 的需求增加 → X 降价使 Y 需求增加 → Y 是 X 的互补品。

2.7-102021 AP Micro 国际卷 Q29

Price	Quantity demanded	Quantity supplied
$1	500	100
$2	400	200
$3	300	300
$4	200	400
$5	100	500

The table provided shows the market demand and supply schedules for wheat. Which of the following is true?

At a price of $1, there is excess supply of wheat and the price will decrease.
At a price of $2, there is excess demand for wheat and the price will decrease.
At a price of $3, there is excess demand for wheat and the price will increase.
At a price of $4, there is excess supply of wheat and the price will decrease.
At a price of $5, there is excess demand for wheat and the price will increase.

$4 时 Qs = 400 > Qd = 200，供给过剩，价格会下降。$3 是均衡；$1、$2 时是需求过剩（短缺），价格上升；$5 时是供给过剩。

2.8Government Intervention in Markets 政府干预

2.6 证明了：没有市场失灵时，竞争市场的均衡让总剩余最大。那政府为什么还要干预？往往是为了公平（让穷人租得起房、让工人工资不太低）或者筹钱（征税）。代价是：在一个本来有效率的市场里，任何价格、数量干预或税收补贴都只会降低配置效率，产生 deadweight loss（无谓损失）（CED POL-1.A.4–5）。

定义 DEFINITION Deadweight loss (DWL): the reduction in total economic surplus that results when a market produces a quantity different from the efficient (equilibrium) quantity. 它是买卖双方损失掉、却没有转移给任何人（包括政府）的那部分剩余。

读懂“字母面积图”

AP 很爱用字母标出图中的每一块面积，再问政策前后 CS、PS、税收、DWL 各是哪几块。先熟悉这张基准图：

六块区域的面积（单位：$）：A = 8，B = 8，C = 2，D = 8，E = 2，F = 8。没有干预时，CS = A + B + C = 18，PS = D + E + F = 18。

Price Ceiling 价格上限

定义 DEFINITION A price ceiling is a legal maximum price. It is binding only if set below the equilibrium price; then it creates a shortage. 设在均衡价格以上的上限不起作用（non-binding）。

价格上限 $4（低于均衡）：只卖得出 4 单位（由供给决定）。CS = A + B + D（蓝），PS = F（绿），C + E 成为无谓损失（红）。

🏠

租金管制（rent control）：房租被压低，想租房的人更多，房东却不愿出租甚至改作他用 → 一房难求、排长队、房屋缺乏维护、出现“中介费”“黑市”。

😷

紧急时期的限价：口罩限价后，正规渠道总是缺货，二手平台上的价格反而更高。

Price Floor 价格下限

定义 DEFINITION A price floor is a legal minimum price. It is binding only if set above the equilibrium price; then it creates a surplus. 例：最低工资（劳动力过剩 = 失业）、农产品保护价（政府往往要自己收购多出来的粮食）。

价格下限 $8（高于均衡）：只买得走 4 单位（由需求决定）。CS = A，PS = B + D + F，C + E 为无谓损失。

WORKED EXAMPLE 11用字母回答：上限和下限原创 · AP 经典考法

Using the lettered graph above (D: P = 12 − Q, S: P = Q), identify consumer surplus, producer surplus, and deadweight loss (a) at equilibrium, (b) with a price ceiling of $4, and (c) with a price floor of $8. Calculate each value.

	Equilibrium	Ceiling $4	Floor $8
Quantity traded	6	4（Qs）	4（Qd）
CS	A + B + C = 18	A + B + D = 24	A = 8
PS	D + E + F = 18	F = 8	B + D + F = 24
DWL	0	C + E = 4	C + E = 4
Total surplus	36	32	32

规律：上限把 D 从生产者转给消费者，下限把 B 从消费者转给生产者；两种情况下 C 和 E 都消失了（交易量从 6 降到 4）。注意上限时 CS 不一定增加——它得到 D 却失去 C，要比较大小。

Per-Unit (Excise) Tax 从量税

对每卖出一单位征收固定金额 t（例如每包烟 $3）。对卖家征税时，卖家要求的价格每单位都高出 t，所以供给曲线向上平移 t；对买家征税则是需求曲线向下平移 t——两种做法的结果完全相同（CED POL-1.A.2）。

对卖家征收每单位 $3 的税：供给曲线向上平移 $3。买家付 $7.5，卖家实收 $4.5，数量降到 4.5。税收收入 = B + D（黄），无谓损失 = C + E（红）。

WORKED EXAMPLE 12每单位征税 $3 原创

The government imposes a $3 per-unit tax on sellers in the market above. Find (a) the new quantity, the price buyers pay, and the price sellers keep; (b) tax revenue and how much of the tax each side bears per unit; (c) consumer surplus, producer surplus, and deadweight loss after the tax.

(a) 新供给：P = Q + 3。与需求 12 − Q 相交：Q = 4.5，买家付 Pc = $7.5，卖家实收 Pp = 7.5 − 3 = $4.5。

(b) 税收收入 = t × Q = 3 × 4.5 = $13.5（B + D）。买家每单位多付 7.5 − 6 = $1.5，卖家每单位少得 6 − 4.5 = $1.5——这里两条曲线一样陡，所以各担一半。

(c) CS = ½ × 4.5 × 4.5 = 10.125（A）；PS = ½ × 4.5 × 4.5 = 10.125（F）；DWL = ½ × 3 × 1.5 = 2.25（C + E）。验算：10.125 + 10.125 + 13.5 + 2.25 = 36 ✓。

Tax Incidence 税负归宿

核心规律（CED POL-1.A.6）税由谁承担，与“法律规定向谁收”无关，只取决于弹性：哪一方更缺乏弹性（更难逃走），哪一方承担更多。

需求缺乏弹性：买家付的价格涨到 Pc，几乎涨了整个税额——税主要由买家承担（深黄 = 买家负担，浅黄 = 卖家负担）。

需求富有弹性：买家一涨价就不买了，卖家只好自己吸收大部分税——税主要由卖家承担。

🚬

香烟税：烟民的需求非常缺乏弹性，税几乎全部转嫁到烟价上，由烟民承担。

🛥️

游艇奢侈税：买家可以改买别的奢侈品（需求富有弹性），结果游艇销量大跌，造游艇的工人和企业承担了大部分税负。

两个极端需求完全无弹性（垂直）或供给完全有弹性（水平）→ 税全部由买家承担；供给完全无弹性或需求完全有弹性 → 税全部由卖家承担。另外，只有一条曲线完全无弹性时，交易量不变，没有无谓损失。

Per-Unit Subsidy 补贴

每单位补贴 $3：供给曲线向下平移 $3。买家只付 $4.5，卖家实收 $7.5，数量增加到 7.5。政府支出（黄色矩形）大于 CS 与 PS 的增加量，差额就是无谓损失：生产了 MC > MB 的单位。

WORKED EXAMPLE 13每单位补贴 $3 原创

Instead of a tax, the government gives sellers a $3 per-unit subsidy. Find the new quantity, the prices buyers pay and sellers receive, the government's cost, and the deadweight loss.

新供给：P = Q − 3。与 12 − Q 相交：Q = 7.5，买家付 $4.5，卖家实收 4.5 + 3 = $7.5。

政府支出 = 3 × 7.5 = $22.5。CS = ½ × 7.5 × 7.5 = 28.125（增加 10.125），PS = 28.125（增加 10.125），两者合计只增加 20.25 < 22.5。

DWL = 22.5 − 20.25 = $2.25 = ½ × 3 × 1.5。原因：第 6 到 7.5 单位的 MC 高于 MB，本不该生产。

Quantity Controls (Quotas) 数量管制

配额限制只能卖 4 单位：买家愿意付 $8，而第 4 单位的成本只有 $4，中间的差价（紫色）落到持有配额许可证的人手里，叫 quota rent；红色三角形是无谓损失。

例子 · 出租车牌照很多城市限制出租车牌照数量（quota）。乘客愿意付的价格高于司机的成本，差价变成了牌照的价值——有的城市一块牌照曾经卖到上百万。配额和价格管制一样会造成无谓损失。

LAB 5政府干预实验室 · Policy Lab

同一个市场（D：P = 12 − Q，S：P = Q）。选一种政策、拖动力度，看价格、数量和各块面积怎么变。

上限价格：

练习 Practice 2.8

以下为历年 AP 真题，点选项即时判分并显示解析。

2.8-12005 AP Micro Q4

Which of the following will occur if the government imposes a price ceiling below the equilibrium price of a good?

The quantity sold will exceed the equilibrium quantity.
Firms' total revenues will increase if demand is price elastic.
There will be a shortage in the market.
All firms will shut down, since price is below the equilibrium price.
Price will exceed the marginal cost of producing the last unit sold.

有效的价格上限（低于均衡）→ 需求量 > 供给量 → 短缺。(A) 成交量减少；(E) 成交量少于均衡，最后一单位的 MC 低于买家愿付的价格。

2.8-22010 AP Micro Q22

After the government imposed a $0.20 per gallon tax on gasoline, the price of a gallon of gasoline increased from $1.00 to $1.15. Which of the following statements is true?

Consumers bear the entire burden of the tax, since producers can pass the tax along to consumers.
Consumers and producers share the tax burden equally.
Consumers bear most, but not all, of the tax burden.
Producers bear the entire burden of the tax, since the tax was levied on producers, not consumers.
There is no tax burden, since gasoline is a normal good.

税 $0.20 中，消费者多付了 $0.15，生产者负担 $0.05 → 消费者承担大部分，但不是全部。

2.8-32012 AP Micro Q48

Suppose that the market supply curve for shoes is upward sloping and the market demand curve is downward sloping. How will the imposition of a sales tax on shoes affect the consumer surplus, the producer surplus, and the total surplus?

Consumer Surplus: Decrease；Producer Surplus: Decrease；Total Surplus: Decrease
Consumer Surplus: Decrease；Producer Surplus: Increase；Total Surplus: Increase
Consumer Surplus: Decrease；Producer Surplus: Increase；Total Surplus: Decrease
Consumer Surplus: Increase；Producer Surplus: Decrease；Total Surplus: Decrease
Consumer Surplus: Increase；Producer Surplus: Increase；Total Surplus: Increase

从量税（销售税）使价格上升、数量减少：CS 和 PS 都下降，总剩余也下降（税收收入弥补不了无谓损失）。

2.8-42014 AP Micro 练习卷 Q9

If the demand for a good is perfectly price inelastic in the short run and the supply curve is upward sloping, imposing a sales tax on the good will

leave the price paid by consumers unchanged
decrease the after-tax revenues received by suppliers
increase the after-tax revenues received by suppliers
not change the after-tax revenues received by suppliers
not change the total expenditures by consumers on the good

需求完全无弹性：税全部转嫁给消费者，价格上涨的幅度等于税额，数量不变 → 卖家税后得到的价格和收入都不变。

2.8-52016 AP Micro 练习卷 Q2

If an effective rent ceiling is eliminated, which of the following is most likely to occur in the rental housing market?

An increase in the demand for housing, resulting in a decrease in the quantity of housing supplied
An increase in rents, resulting in an increase in the quantity of housing supplied
An increase in the demand for housing, resulting in an increase in the quantity of housing demanded
A decrease in rents, resulting in an increase in the quantity of housing supplied
A decrease in the demand for housing, resulting in an increase in the quantity of housing supplied

取消有效的租金上限 → 租金上升到均衡水平 → 房东愿意出租的数量（供给量）增加。需求曲线本身不移动。

2.8-62017 AP Micro 练习卷 Q24

Assume that the current market equilibrium price for milk is $2.80 per gallon and that 5 million gallons are sold per day. If the government sets a price ceiling of $2.00 per gallon, which of the following is true?

The demand for milk will increase.
The supply of milk will decrease.
There will be an excess supply of milk in the market.
More than 5 million gallons of milk will be sold.
Less than 5 million gallons of milk will be sold.

上限 $2.00 低于均衡价 $2.80 → 供给量减少，成交量 = 供给量 < 500 万加仑（同时出现短缺）。供给曲线和需求曲线都不移动。

2.8-72017 AP Micro 练习卷 Q54

Assume that the government imposes an excise tax on the consumption of a good. The tax will have the least impact on the market equilibrium quantity for which of the following combinations of the price elasticities of demand and supply?

Absolute Value of Elasticity of Demand: 4；Elasticity of Supply: 4
Absolute Value of Elasticity of Demand: 4；Elasticity of Supply: 0.50
Absolute Value of Elasticity of Demand: 1；Elasticity of Supply: 1
Absolute Value of Elasticity of Demand: 0.50；Elasticity of Supply: 4
Absolute Value of Elasticity of Demand: 0.50；Elasticity of Supply: 0.50

需求和供给都缺乏弹性时，税对数量的影响最小（两条曲线越陡，数量越难变化）。

2.8-82018 AP Micro 练习卷 Q34

Assume that the supply of corn is relatively price inelastic, while the demand for corn is relatively price elastic. If the government imposes a per-unit excise tax on the production of corn, the incidence of the tax will fall

entirely on buyers
more on buyers than on sellers
entirely on sellers
more on sellers than on buyers
equally on both buyers and sellers

供给相对缺乏弹性、需求相对富有弹性 → 税负主要落在卖家身上。规律：谁更缺乏弹性，谁负担更多。

2.8-92021 AP Micro 国际卷 Q25

If the imposition of a per-unit tax on a good reduces the total consumer surplus by more than the reduction of total producer surplus in a competitive market, then which of the following must be true?

The price sellers receive decreases by more than the amount of the tax.
The price buyers pay increases by more than the amount of the tax.
Demand is less elastic than supply.
Demand is more elastic than supply.
Demand and supply are both unit elastic.

CS 减少得比 PS 多 → 消费者承担了更多税负 → 需求比供给更缺乏弹性。(A)(B) 两种价格的变化之和恰好等于税额，不可能有一方超过税额。

2.8-102021 AP Micro 国际卷 Q52

Which of the following will result in the short run if the government imposes a binding quota?

A decrease in the price paid by consumers
A rightward shift in the supply curve
A leftward shift in the demand curve
An increase in consumer surplus
A deadweight loss

有约束力的配额把数量限制在均衡数量以下 → 价格上升、消费者剩余减少，产生无谓损失。曲线本身不移动。

2.8-112019 AP Micro 练习卷 Q17

按原题重绘

Assume that the government imposes a $4 per-unit tax on sellers of a good in the market described by the graph above. What are the price paid by buyers, the after-tax price received by sellers, and the deadweight loss?

Price Paid by Buyers: $8；Price Received by Sellers: $6；Deadweight Loss: $100
Price Paid by Buyers: $8；Price Received by Sellers: $4；Deadweight Loss: $200
Price Paid by Buyers: $6；Price Received by Sellers: $6；Deadweight Loss: $0
Price Paid by Buyers: $4；Price Received by Sellers: $8；Deadweight Loss: $100
Price Paid by Buyers: $4；Price Received by Sellers: $8；Deadweight Loss: $200

税后供给：P = 6 + 0.02Q；与需求 P = 10 − 0.02Q 相交于 Q = 100。买家付 $8，卖家实收 8 − 4 = $4。DWL = ½ × 4 × (200 − 100) = $200。

2.8-122022 AP Micro 国际卷 Q17

按原题重绘

The graph provided shows the imposition of a per-unit tax on producers. Which areas represent government tax revenue, producer surplus, and deadweight loss after the imposition of the tax?

The government tax revenue is P5P3MJ; The producer surplus is P3P1L; The deadweight loss is JNL.
The government tax revenue is P5P2NJ; The producer surplus is P6P5J; The deadweight loss is P4P5J.
The government tax revenue is JNL; The producer surplus is P3P1NM; The deadweight loss is MLN.
The government tax revenue is P5P2NJ; The producer surplus is P2P1N; The deadweight loss is JNL.
The government tax revenue is P5P3MJ; The producer surplus is P3P1L; The deadweight loss is JML.

税后数量 Q₁：税收收入 = 税额（P₅ − P₂）× Q₁ = P₅P₂NJ；卖家实收 P₂，PS = 供给曲线以上、P₂ 以下 = P₂P₁N；DWL = 税前税后均衡之间的三角形 JNL。

2.9International Trade & Public Policy 国际贸易与贸易政策

把 1.4 的比较优势和这一单元的供需图结合起来：一个国家开放贸易后，国内价格变成世界价格（world price）。如果这个国家只是世界市场上的小买家/小卖家（price taker），世界价格就是一条水平线。

两种情况（CED POL-1.B.1）

Pw < 国内均衡价 → 国内价格下降到 Pw → Qd > Qs → 进口 = Qd − Qs。消费者获益，国内生产者受损，总剩余增加。
Pw > 国内均衡价 → 国内价格上升到 Pw → Qs > Qd → 出口 = Qs − Qd。生产者获益，国内消费者受损，总剩余增加。

世界价格 $4 低于国内均衡价 $6：国内价格降到 $4，国内生产 4、消费 8，差额 4 靠进口。CS 由 A 增加到 A + B + C + D；PS 由 B + C + F 减少到 F；总剩余净增加 D。

WORKED EXAMPLE 14开放进口牛油果原创

A country's domestic avocado market has D: P = 12 − Q and S: P = Q (Q in thousand tons). The country opens to trade at a world price of $4. Find (a) domestic quantity supplied, quantity demanded, and imports; (b) CS and PS before and after trade; (c) the change in total surplus.

(a) P = 4：Qs = 4，Qd = 12 − 4 = 8，进口 = 4。

(b) 贸易前 CS = 18，PS = 18。贸易后 CS = ½ × 8 × (12 − 4) = 32，PS = ½ × 4 × 4 = 8。

(c) 总剩余从 36 增加到 40，净增加 4（区域 D）。B + C = 10 只是从生产者转移给消费者。

Tariffs & Quotas 关税与进口配额

定义 DEFINITION A tariff is a tax on imported goods; an import quota is a legal limit on the quantity of a good that can be imported. 两者都是 protectionism（贸易保护）的手段，目的是保护国内生产者。

征收 $1 关税：国内价格升到 $5，国内生产增加到 5、消费减少到 7，进口降到 2。黄色矩形 = 关税收入；两个红色三角形 = 无谓损失。

WORKED EXAMPLE 15对进口牛油果征收 $1 关税原创

Continuing the example, the government imposes a $1 per-unit tariff on avocado imports. Find the new domestic price, domestic production, consumption, imports, tariff revenue, and deadweight loss.

国内价格 = Pw + t = $5。Qs = 5，Qd = 7，进口 = 2。

关税收入 = 1 × 2 = $2。CS = ½ × 7 × 7 = 24.5（减少 7.5）；PS = ½ × 5 × 5 = 12.5（增加 4.5）。

DWL = 两个三角形 = ½ × 1 × 1 + ½ × 1 × 1 = $1：左边是“让效率较低的国内生产者多生产”的损失，右边是“消费者少买”的损失。验算：24.5 + 12.5 + 2 = 39 = 40 − 1 ✓。

进口配额 vs 关税（CED POL-1.B.3）配额把进口量限制在某个数，同样会让国内价格上升、国内生产增加、CS 下降、产生 DWL。区别：配额不给政府带来收入，原本的“关税收入”那块变成了拿到进口许可证的人的 quota rent。考纲规定不考配额的作图，但要知道它对数量、价格和剩余的影响。

为什么明明有无谓损失，很多国家还要保护？关税的好处集中在少数国内生产者身上（每家得到很多，有动力去游说），而损失分散在千千万万消费者身上（每人只多花一点，不在意）。另外还有就业、国家安全、保护新兴产业等理由。AP 考的是图上的效果，这些理由了解即可。

LAB 6国际贸易与关税 · Trade Lab

国内市场 D：P = 12 − Q，S：P = Q（不贸易时 P = $6）。调整世界价格和关税。

世界价格 Pw = $4

每单位关税 = $0（只在进口时起作用）

练习 Practice 2.9

以下为历年 AP 真题，点选项即时判分并显示解析。

2.9-12013 AP Micro Q4

If the government imposes a tariff on imports of cheese, the price and quantity of imported cheese will most likely change in which of the following ways?

Price: Decrease；Quantity: Decrease
Price: Decrease；Quantity: Increase
Price: Increase；Quantity: Not change
Price: Increase；Quantity: Decrease
Price: Increase；Quantity: Increase

关税使进口奶酪在国内的价格上升，进口数量减少。

2.9-22023 AP Micro 国际卷 Q16

A nation engages in international trade with its trading partners and is currently an importer of aluminum. Imposing an import tariff on aluminum will affect the domestic market for aluminum in which of the following ways?

Domestic Quantity Supplied: Increase；Domestic Consumer Surplus: Decrease；Domestic Producer Surplus: Decrease
Domestic Quantity Supplied: Increase；Domestic Consumer Surplus: Decrease；Domestic Producer Surplus: Increase
Domestic Quantity Supplied: No change；Domestic Consumer Surplus: Decrease；Domestic Producer Surplus: Decrease
Domestic Quantity Supplied: Decrease；Domestic Consumer Surplus: Increase；Domestic Producer Surplus: Decrease
Domestic Quantity Supplied: Decrease；Domestic Consumer Surplus: Increase；Domestic Producer Surplus: Increase

对进口品征关税 → 国内价格上升 → 国内供给量增加，CS 减少，国内 PS 增加。

2.9-32025 AP Micro 国际卷 Q22

按原题重绘

The graph shows the domestic demand for rice and the domestic supply of rice in Deltaland.

If rice is freely traded in the world and the world price of rice is $3, then Deltaland will

export 4 million pounds of rice
export 8 million pounds of rice
import 4 million pounds of rice
import 8 million pounds of rice
import 12 million pounds of rice

Pw = $3：国内需求 9 − 0.5Q = 3 → Qd = 12；国内供给 1 + 0.5Q = 3 → Qs = 4。进口 = 12 − 4 = 8（百万磅）。

2.9-42025 AP Micro 国际卷 Q44

Which of the following describes a difference between the imposition of a tariff on imports and the imposition of a quota on imports?

A tariff will affect domestic consumption, whereas a quota will not.
A tariff will affect domestic production, whereas a quota will not.
A tariff will increase domestic consumer surplus, whereas a quota will not.
A tariff will generate revenue for the government, whereas a quota will not.
A tariff will eliminate deadweight loss, whereas a quota will not.

关税和配额都会提高国内价格、减少消费、增加国内生产、产生无谓损失；区别是只有关税给政府带来收入，配额的这部分收益归进口许可证持有者。

ΣSummary 单元总结与易错点

2.1 Demand

需求定律：P 与 Qd 反向；原因：替代效应、收入效应、边际效用递减。市场需求 = 个人需求横向相加。自身价格 → 沿曲线移动；TRIBE（偏好、相关商品价格、收入、买家数量、预期）→ 曲线平移。

2.2 Supply

供给定律：P 与 Qs 同向。ROTTEN（投入品价格、其他商品价格、技术、税与补贴、预期、卖家数量）→ 供给平移；供给增加 = 右移（看起来向下）。

2.3 PED

|%ΔQd ÷ %ΔP|，中点法。>1 elastic，<1 inelastic，=1 unit。决定因素：替代品、必需/奢侈、占收入比例、时间、市场定义。线性需求：上半段 elastic、中点 unit、下半段 inelastic；斜率 ≠ 弹性。总收益检验：inelastic 时 P 与 TR 同向，elastic 时反向。

2.4 PES

%ΔQs ÷ %ΔP。时间越长、投入越容易获得和转换、有闲置产能和库存 → 越有弹性。垂直供给 PES = 0。

2.5 YED & XED

YED > 0 正常品（>1 奢侈品，0–1 必需品），< 0 劣等品。XED > 0 替代品，< 0 互补品，≈ 0 无关。正负号有意义，不能取绝对值。

2.6 Equilibrium & Surplus

Qd = Qs。CS = WTP − 价格（需求曲线下、价格上）；PS = 价格 − 成本（价格下、供给曲线上）。竞争均衡使总剩余最大 → 有效率。

2.7 Changes

价格低于均衡 → 短缺；高于均衡 → 过剩。D↑：P↑Q↑；D↓：P↓Q↓；S↑：P↓Q↑；S↓：P↑Q↓。双移动时 P、Q 中一个不确定。变化幅度取决于弹性。关联市场：投入品影响供给，替代/互补品影响需求。

2.8 Government Intervention

有效上限（低于均衡）→ 短缺；有效下限（高于均衡）→ 过剩；成交量取两者中较小的。从量税：S 上移 t，税收 = t × Q，DWL 三角形；谁更缺乏弹性谁承担更多。补贴：S 下移，政府支出 > 剩余增加。配额：数量受限，产生 quota rent 与 DWL。

2.9 Trade

Pw < 国内均衡 → 进口 = Qd − Qs，CS↑ PS↓ 总剩余↑；Pw > 国内均衡 → 出口。关税：国内价格 ↑ 到 Pw + t，进口↓，政府收入，两个 DWL 三角形。配额：效果类似但没有政府收入。

易错点 TOP 10

“价格上涨使需求减少” ✗ → 自身价格变化只改变需求量。
供给增加往上画 ✗ → 供给增加是右移（看起来向下）。
收入、偏好变化移动供给曲线 ✗ → 它们影响需求；投入品价格影响供给。
用斜率判断同一条直线上的弹性 ✗ → 线性需求曲线上弹性一路在变。
“涨价一定增加收入” ✗ → 只有需求缺乏弹性时才成立。
YED、XED 取绝对值 ✗ → 正负号决定商品类型。
短缺发生在价格高于均衡时 ✗ → 价格低于均衡才短缺。
双移动时两个变量都能确定 ✗ → 一定有一个“不确定”。
“对卖家征税，卖家承担全部税负” ✗ → 税负只取决于弹性。
价格上限设在均衡价格以上也会造成短缺 ✗ → 不起作用（non-binding）。

✎Unit 2 Test 单元测验

20 道选择题，全部选自历年 AP 真题（与各节练习不重复），建议用时 24 分钟。先全部选完，再点底部“交卷”查看得分和解析。

T12014 AP Micro 练习卷 Q4

Assume that ice cream is a normal good. If the price of ice cream decreases, the substitution effect and the income effect will lead to which of the following changes in ice cream consumption?

Substitution Effect: Increase；Income Effect: Decrease
Substitution Effect: Increase；Income Effect: Increase
Substitution Effect: Increase；Income Effect: No change
Substitution Effect: Decrease；Income Effect: Increase
Substitution Effect: Decrease；Income Effect: No change

冰淇淋降价：相对其他商品更便宜 → 替代效应使消费增加；购买力上升且冰淇淋是正常品 → 收入效应也使消费增加。

T22016 AP Micro 练习卷 Q35

Which of the following will cause the demand curve for good X to shift to the right?

An increase in the price of good Z, a complement to good X
An increase in the price of good Y, a substitute for good X
An increase in the consumer’s income, if good X is an inferior good
A decrease in the price of good X
An increase in the supply of good X

替代品 Y 涨价 → X 的需求右移。(A) 互补品涨价让需求左移；(C) 劣等品收入增加需求左移；(D) 自身价格；(E) 供给。

T32000 AP Micro Q20

Which of the following must be true if the revenues of wheat farmers increase when the price of wheat increases?

The supply of wheat is price elastic.
The supply of wheat is income elastic.
The supply of wheat is income inelastic.
The demand for wheat is price elastic.
The demand for wheat is price inelastic.

涨价后农民收入增加 → 价格与 TR 同方向 → 小麦需求缺乏弹性。

T42012 AP Micro Q33

If a severe drought destroys a significant portion of the peanut crop and peanut farmers’ revenues increase, which of the following is true over the observed range of prices?

The demand for peanuts must be unit price elastic.
The demand for peanuts must be price elastic.
The demand for peanuts must be price inelastic.
The supply of peanuts must be price inelastic.
The supply of peanuts must be price elastic.

干旱减少供给、价格上升，而农民收入增加 → 价格与 TR 同方向 → 需求缺乏弹性（这正是“谷贱伤农”的反面）。

T52021 AP Micro 国际卷 Q45

If the absolute value of the price elasticity of demand for Good X is 0.5, then a 10 percent decrease in the price of Good X will result in which of the following?

A 5% decrease in the quantity demanded of Good X
A 5% increase in the quantity demanded of Good X
A 5% increase in revenues from the sale of Good X
A 10% decrease in revenues from the sale of Good X
A 10% increase in revenues from the sale of Good X

%ΔQd = 0.5 × 10% = 5%，降价 → 需求量增加 5%。需求缺乏弹性，降价后收入会减少（大约 5%），所以 (C)(E) 错。

T62022 AP Micro 国际卷 Q11

The cross-price elasticity of demand for Goods X and Y is −2.8. Based on this information, what is true about Good X and Good Y?

Good X is a normal good and Good Y is an inferior good.
The goods are inferior.
The goods are substitutes.
The goods are complements.
The goods have upward-sloping demand curves.

交叉弹性为负 → 互补品。交叉弹性本身说明不了正常品/劣等品（那是收入弹性）。

T72025 AP Micro 国际卷 Q27

If the income elasticity of demand for Good X is negative, how will an increase in consumers’ income affect the market for Good X?

The demand curve will shift to the left, the equilibrium price will increase, and the equilibrium quantity will decrease.
The demand curve will shift to the left, and both the equilibrium price and quantity will decrease.
The demand curve will shift to the right, and both the equilibrium price and quantity will increase.
The supply curve will shift to the right, the equilibrium price will decrease, and the equilibrium quantity will increase.
The supply curve will shift to the left, the equilibrium price will increase, and the equilibrium quantity will decrease.

收入弹性为负 → 劣等品 → 收入增加使需求左移 → 价格和数量都下降。

T82014 AP Micro 练习卷 Q19

The difference between what consumers are willing to pay for units of a good and the price consumers actually pay for units of the good is called

marginal utility
producer surplus
consumer surplus
economic rent
a positive externality

这正是消费者剩余的定义：愿意支付的价格与实际支付价格之差。

T92023 AP Micro 国际卷 Q31

The market for Good Z has relatively elastic supply and demand curves. Market efficiency is attained when

total labor cost is minimized for each firm
each firm experiences economies of scale
consumer surplus is greater than producer surplus
producer surplus is zero
deadweight loss is zero

市场效率 = 总剩余最大 = 没有无谓损失。CS 与 PS 的相对大小、单个企业的成本都不是效率的判断标准。

T102017 AP Micro 练习卷 Q2

At a price of $10, the quantity demanded of pizzas is 100 and the quantity supplied is 150. Which of the following statements must be true?

There is a shortage of pizza at a price of $10.
The equilibrium price of pizza is below $10.
The quantity of pizzas sold will be 150.
At equilibrium, fewer than 100 pizzas will be sold.
The pizza market is in equilibrium.

$10 时供给量 150 > 需求量 100，过剩 → 价格会下降 → 均衡价格低于 $10。(D) 价格下降时需求量会超过 100，所以均衡数量大于 100。

T112022 AP Micro 国际卷 Q14

Ice cream is a normal good. Ice cream and hot fudge are complementary goods, while ice cream and gelato are substitute goods. Which of the following could lead to an increase in the equilibrium price and an indeterminate change in the equilibrium quantity of ice cream?

Consumer incomes rise, and the price of hot fudge increases.
Bacteria in ice cream facilities leads to a number of consumers being hospitalized, and increased dairy production leads to lower input costs for ice cream.
The price of gelato increases, and new technology increases efficiency of ice cream production.
A new study indicates that ice cream is associated with improved bone health, and a leading ice cream producer declares bankruptcy.
A newly popular diet includes avoiding dairy products, and the government begins to subsidize ice cream production.

(D) 骨骼健康研究 → 需求增加（P↑ Q↑）；大厂破产 → 供给减少（P↑ Q↓）→ 价格一定上升，数量不确定。(A) 收入增加与热巧克力酱涨价对需求的作用相反，价格不确定。

T122022 AP Micro 国际卷 Q22

Which of the following will likely result in an indeterminate change in the equilibrium quantity of a competitive market?

An increase in supply along a perfectly inelastic demand curve
A decrease in supply along a relatively elastic demand curve
A decrease in demand along a perfectly elastic supply curve
An increase in demand and a decrease in supply
An increase in demand and an increase in supply

需求增加（Q↑）与供给减少（Q↓）对数量的影响方向相反 → 数量不确定。(E) 两者都增加时数量一定增加。

T132022 AP Micro 国际卷 Q55

A decrease in the supply of pencils results in which of the following?

A surplus of pencils in the market
An increase in the demand for erasers, a complement in consumption
An increase in the quantity demanded of pencils
A decrease in the consumer surplus of pencil buyers
A decrease in the demand for pencils

铅笔供给减少 → 价格上升、数量减少 → 消费者剩余减少。(B) 铅笔涨价会让互补品橡皮的需求减少；(C) 需求量减少；(E) 需求本身不变。

T142025 AP Micro 国际卷 Q18

An air-conditioning unit is a normal good. Which of the following would cause an increase in the equilibrium price and equilibrium quantity of air-conditioning units in the short run?

Consumers expect a greater increase in the future price of air-conditioning units.
Producers expect a greater increase in the future price of air-conditioning units.
The price of air filters, an input in producing air-conditioning units, increases.
The price of fans, a substitute in consumption, decreases.
Consumers’ incomes decrease.

消费者预期未来价格上涨 → 现在多买 → 需求增加 → P↑ Q↑。(B) 生产者预期涨价会减少当前供给（P↑ Q↓）；(C) 供给减少；(D)(E) 需求减少。

T152005 AP Micro Q48

If the market demand for a good is inelastic and the supply is elastic, which of the following is true when there is an increase in sales tax?

Consumers will bear most of the burden of the tax.
Producers will bear all of the burden of the tax.
Producers will bear most of the burden of the tax or risk losing sales.
Both consumers and producers will share the burden of the tax equally.
The price of the good will not change.

需求缺乏弹性、供给富有弹性 → 消费者承担大部分税负。

T162018 AP Micro 练习卷 Q17

In the rental housing market, what would be the effect of a rent control that was set at the market equilibrium rental price?

It would increase the number of housing units available, creating a surplus.
It would decrease the number of housing units available, creating a shortage.
It would have no impact on the number of housing units available.
It would increase the consumer surplus and decrease the producer surplus.
It would decrease the consumer surplus and increase the producer surplus.

租金上限设在均衡价格上，不起作用（non-binding），市场不受影响。

T172019 AP Micro 练习卷 Q48

The equilibrium price for a good with a vertical supply curve and a downward-sloping demand curve is $20. If a binding price floor is set, which of the following will occur?

There will be a shortage of the good.
The sum of consumer and producer surpluses will decrease.
The equilibrium price of the good will decrease.
The quantity sold of the good will remain unchanged.
Demand for substitutes for the good will decrease.

供给垂直、设置有效的价格下限：价格被抬高，需求量下降 → 成交量减少、出现过剩，总剩余减少。(D) 错在成交量由需求量决定，会减少。

T182022 AP Micro 国际卷 Q56

The fresh fish market is in equilibrium, and the demand is perfectly elastic. The imposition of a per-unit tax on sellers by the government will result in which of the following?

The equilibrium price will increase by the full amount of the tax, and consumers will bear the entire burden of the tax.
The equilibrium price will remain unchanged, and consumers will bear the entire burden of the tax.
The equilibrium price will increase by the full amount of the tax, and suppliers will bear the entire burden of the tax.
The equilibrium price will remain unchanged, and suppliers will bear the entire burden of the tax.
The equilibrium price will increase by the full amount of the tax, and suppliers and consumers will bear the burden of the tax equally.

需求完全有弹性（水平）：价格不能变，税全部由卖家承担，均衡价格不变、数量减少。

T192025 AP Micro 国际卷 Q10

If the government imposes a binding price ceiling in the gasoline market, which of the following will occur in the short run?

The demand for gasoline will increase.
The demand for gasoline will decrease.
The quantity demanded of gasoline will increase.
All individuals in the gasoline market will be better off.
All individuals in the gasoline market will be worse off.

有效的价格上限使价格低于均衡 → 需求量增加（沿需求曲线），同时供给量减少，出现短缺。需求曲线本身不动；有的人获益（买到便宜货），有的人受损。

T202024 AP Micro Q23

Country X imports tea from its trading partners. The government is evaluating whether it should impose a tariff or a quota in the market for tea in Country X. Which of the following statements is true regarding the difference in the outcomes between these two government interventions in the market for tea?

A tariff will cause a decrease in domestic consumer surplus, while a quota will not.
A tariff will generate government revenue, while a quota will not.
A tariff will cause a decrease in domestic producer surplus, while a quota will not.
A tariff will cause deadweight loss, while a quota will not.
A tariff will increase the price of tea for domestic consumers, while a quota will not.

关税给政府带来收入，配额没有。两者都会提高国内价格、减少 CS、增加国内 PS、产生无谓损失。（本卷答案键为非官方整理，本题已重新核对。）

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FRQFree-Response Practice 自由问答

前四道是 AP 真题，评分标准依照 College Board 官方 Scoring Guidelines 整理；最后一道是原创综合题。先在纸上完整作答（包括画图），再展开对照。

FRQ 1 · 2016 AP Micro FRQ Q1（长题）· 10 分 · 价格下限、弹性、关联市场真题

The markets for bananas, muffins, and coffee are interrelated, and each market is perfectly competitive.

(a) In the market for bananas, the equilibrium price is $1.00 per pound, and the equilibrium quantity is 1,000 pounds per week. Suppose the government imposes a price floor on bananas at $1.20 per pound, causing the quantity supplied to increase to 1,500 pounds per week.
  (i) Would the price floor result in a shortage, a surplus, or neither? Explain.
  (ii) Calculate the price elasticity of supply if the price increases from $1 to $1.20. Show your work.
  (iii) Between $1 and $1.20, is the supply elastic, unit elastic, or inelastic? Explain.
(b) Bananas are an input for muffins.
  (i) Draw a correctly labeled graph of the market for muffins indicating the equilibrium price and quantity, labeled P₀ and Q₀, respectively.
  (ii) On the graph drawn in part (b)(i), show the impact of an increase in the price of bananas on the muffin market, labeling the new equilibrium price and quantity P₁ and Q₁, respectively.
  (iii) On the same graph, completely shade the area that represents the change in the consumer surplus caused by the increase in the price of bananas.
(c) In the market for coffee, the equilibrium price is $3.00 per cup and the equilibrium quantity is 100 cups per week. The cross-price elasticity of coffee with respect to muffins is negative two.
  (i) Are coffee and muffins normal goods, inferior goods, complementary goods, or substitute goods?
  (ii) Assume the supply of coffee is perfectly elastic. Using the equilibrium price and quantity given above, draw a correctly labeled graph for the coffee market, and show the impact of an increase in the price of muffins on the coffee market.
  (iii) Given the original quantity of 100 cups of coffee per week, if the increase in the price of muffins is 10%, calculate the new equilibrium quantity in the coffee market. Show your work.

Point	得分要求与解析
(a)(i) 1 pt	Surplus：下限高于均衡价格，供给量（1,500）超过需求量（需求量会低于 1,000）。
(a)(ii) 1 pt	PES = %ΔQs ÷ %ΔP = 50% ÷ 20% = 2.5（中点法 (500/1,250) ÷ (0.2/1.1) = 2.2 也给分）。
(a)(iii) 1 pt	Elastic：供给量变化的百分比大于价格变化的百分比（PES > 1）。
(b) 3 pts	① 正确标注的供需图，标出 P₀、Q₀；② 香蕉是投入品，香蕉涨价使松饼供给左移，标出新的 P₁ 更高、Q₁ 更少；③ 完整涂出消费者剩余减少的区域。
(c) 4 pts	① XED 为负 → 互补品；② 咖啡市场图：水平供给线 + 向下倾斜的需求线；③ 松饼涨价 → 咖啡需求左移，价格不变（$3）、数量减少；④ %ΔQ = −2 × 10% = −20%，新数量 = 100 × (1 − 0.2) = 80 cups。

松饼市场：香蕉（投入品）涨价 → 供给左移 → P↑ Q↓；消费者剩余减少的部分（P₀ 与 P₁ 之间、需求曲线左侧的梯形）要完整涂满。

咖啡市场：供给完全弹性（水平）；松饼涨价 → 互补品咖啡的需求左移 → 价格不变，数量从 100 降到 80。

FRQ 2 · 2022 AP Micro FRQ Set 1 Q3 · 5 分 · 市场需求与需求弹性真题

In the local market for Good X, there are four individual buyers: Emily, Wu, Omar, and Fernanda. The quantities that each individual buyer would be willing and able to purchase at different prices are included in the table provided.

Price	Emily	Wu	Omar	Fernanda
$1	5	4	4	3
$2	4	3	4	3
$3	4	3	3	2
$4	3	2	3	2
$5	3	2	2	1
$6	2	1	2	1
$7	2	1	1	0
$8	1	0	1	0

(a) The local market for Good X has a perfectly elastic supply. Draw a correctly labeled graph for the local market for Good X with a market equilibrium price of $5. Label the equilibrium price as $5, and label the equilibrium quantity for the market with a specific value based on the data provided in the table.
(b) Assume the cost of production increases, which causes the price of Good X to increase from $5 to $7.
(i) Calculate the price elasticity of demand for Good X as the price increases from $5 to $7. Show your work.
(ii) Identify whether the demand for Good X is elastic, inelastic, or unit elastic in that range of prices.
(c) Could Emily's marginal benefit for the second unit of Good X equal $4.50? Explain.

Point	得分要求与解析
(a) 1 pt	正确标注的图：向下倾斜的需求曲线 D，在 $5 处水平的供给曲线 S。
(a) 1 pt	均衡数量标为 8（$5 时：3 + 2 + 2 + 1 = 8，市场需求是个人需求的横向加总）。
(b)(i) 1 pt	$7 时市场需求量 = 2 + 1 + 1 + 0 = 4。PED = [(4 − 8) ÷ 8] ÷ [(7 − 5) ÷ 5] = −50% ÷ 40% = −1.25（绝对值 1.25）。
(b)(ii) 1 pt	Elastic（\|PED\| > 1）。
(c) 1 pt	No。Emily 在 $7 时仍愿意买 2 单位，说明她对第 2 单位的边际收益至少是 $7，不可能只有 $4.50。

需求向下倾斜、供给在 $5 处水平；$5 时市场需求量 = 3 + 2 + 2 + 1 = 8。

FRQ 3 · 2022 AP Micro FRQ Set 2 Q2 · 5 分 · 剩余与国际贸易（出口）真题

The graph provided depicts New Zealand's domestic supply and demand for wool.

按原题重绘：新西兰羊毛的国内供需（数量单位：units）

(a) Calculate the consumer surplus if New Zealand does not trade with the rest of the world. Show your work.
(b) Instead, assume New Zealand decides to trade wool in the world market. The current world price of wool is $60 per unit, and New Zealand is a price taker in the world market.
  (i) How many units of wool will New Zealand export?
  (ii) What will happen to the consumer surplus of wool consumers in New Zealand when New Zealand begins to trade with the rest of the world? Explain.
  (iii) Will total economic surplus in New Zealand increase, decrease, or remain unchanged when New Zealand begins to trade wool in the world market? Explain using numbers.
(c) Now assume domestic demand in New Zealand increases. Will New Zealand's exports increase, decrease, or stay the same?

Point	得分要求与解析
(a) 1 pt	不贸易时均衡 P = $40、Q = 300。CS = ½ × 300 × (70 − 40) = $4,500。
(b)(i) 1 pt	P = $60：国内供给量 500，国内需求量 100，出口 = 400。
(b)(ii) 1 pt	CS 减少：国内价格升到世界价格，国内需求量减少；CS 从 $4,500 降到 ½ × 100 × 10 = $500。
(b)(iii) 1 pt	总剩余增加 $4,000：PS 从 ½ × 300 × 30 = $4,500 增加到 ½ × 500 × 50 = $12,500（+$8,000），CS 减少 $4,000。
(c) 1 pt	出口减少：世界价格不变，国内需求量增加而国内供给量不变。

FRQ 4 · 2025 AP Micro FRQ Set 1 Q2 · 5 分 · 总剩余、价格下限、贸易真题

The graph provided shows the market for rice in the country of Rushland.

按原题重绘：Rushland 的大米市场

A. Calculate the total economic surplus at market equilibrium. Show your work.
B. If the government sets a price floor at $3 per bushel, will there be a surplus, a shortage, or neither? Explain.
C. Suppose that instead of the price floor, Rushland engages in international trade and the world price of rice is $5 per bushel.
  i. Will Rushland export or import rice? Explain using numbers from the graph.
  ii. Calculate the domestic consumer surplus when Rushland engages in international trade. Show your work.
  iii. Calculate the total revenue that Rushland's farmers will earn at the world price. Show your work.

Point	得分要求与解析
A 1 pt	均衡 P = $4，Q = 60。总剩余 = ½ × (10 − 1) × 60 = $270（或 CS 180 + PS 90）。
B 1 pt	Neither：下限 $3 低于均衡价格 $4，不起作用，价格和数量不变。
C(i) 1 pt	Export：$5 时国内供给量 80 > 国内需求量 50，多出的 30 蒲式耳出口。
C(ii) 1 pt	CS = ½ × (10 − 5) × 50 = $125。
C(iii) 1 pt	TR = 世界价格 × 国内农民卖出的数量 = 5 × 80 = $400（卖给国内和国外的都算）。

FRQ 5 · 综合练习 · 7 分 · 从量税原创

In a perfectly competitive market, demand is P = 14 − Q and supply is P = 2 + Q.

需求 P = 14 − Q，供给 P = 2 + Q

(a) Identify the equilibrium price and quantity.
(b) Calculate consumer surplus at the equilibrium.
(c) The government imposes a per-unit tax of $4 on sellers. Identify the new quantity, the price paid by buyers, and the price received by sellers.
(d) Calculate the tax revenue.
(e) Calculate the deadweight loss.
(f) How much of the tax per unit is paid by buyers?
(g) If demand were more inelastic, would the share of the tax paid by buyers increase, decrease, or stay the same?

Point	得分要求与解析
(a) 1 pt	14 − Q = 2 + Q → Q = 6，P = $8。
(b) 1 pt	CS = ½ × 6 × (14 − 8) = $18。
(c) 1 pt	税后供给 P = 6 + Q；14 − Q = 6 + Q → Q = 4，买家付 $10，卖家实收 10 − 4 = $6。
(d) 1 pt	税收 = 4 × 4 = $16。
(e) 1 pt	DWL = ½ × 4 × (6 − 4) = $4。
(f) 1 pt	买家每单位多付 10 − 8 = $2（卖家少得 8 − 6 = $2）。
(g) 1 pt	Increase：需求越缺乏弹性，买家越难减少购买，税负越多地落在买家身上。

FRQ 答题提醒

供需图：纵轴 Price、横轴 Quantity；曲线标 D、S；原均衡标 P₁/Q₁，新均衡标 P₂/Q₂；移动用箭头或新曲线 D₂/S₂。
“Show your work” 要写出算式：三角形面积 ½ × 底 × 高，弹性写出两个百分比。
“Shade completely” 要把整块区域涂满，只画边框不给分。
“Explain” 要说出原因：例如“下限低于均衡价格，所以不起作用”。

JL · AP Microeconomics · Unit 2 Supply and Demand · 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

Unit 2 · Supply and Demand

2.1Demand 需求

Demand Schedule & Demand Curve 需求表与需求曲线

Why Does Demand Slope Downward? 为什么向右下倾斜

Individual vs. Market Demand 个人需求与市场需求

Movement Along vs. Shift of the Curve 沿曲线移动 vs. 曲线平移

Determinants of Demand 需求的决定因素（让 D 平移的原因）

练习 Practice 2.1

2.2Supply 供给

Determinants of Supply 供给的决定因素（让 S 平移的原因）

练习 Practice 2.2

2.3Price Elasticity of Demand 需求的价格弹性

The Midpoint Method 中点法

Five Ranges of PED 五种情况

Determinants of PED 影响需求弹性的因素（CED MKT-3.E.4）

Elasticity Along a Linear Demand Curve & Total Revenue 线性需求曲线与总收益

中点法计算器 Midpoint Calculator

练习 Practice 2.3

2.4Price Elasticity of Supply 供给的价格弹性

Determinants of PES 影响供给弹性的因素

练习 Practice 2.4

2.5Other Elasticities 其他弹性：收入弹性与交叉弹性

练习 Practice 2.5

2.6Market Equilibrium & Consumer/Producer Surplus 市场均衡与剩余

Consumer Surplus 消费者剩余

Producer Surplus 生产者剩余

Surplus in a Market Graph 市场图中的剩余

练习 Practice 2.6

2.7Disequilibrium & Changes in Equilibrium 失衡与均衡的变动

Shortage & Surplus 短缺与过剩

One Curve Shifts 单条曲线移动

Both Curves Shift 双移动

Related Markets 关联市场的连锁反应

练习 Practice 2.7

2.8Government Intervention in Markets 政府干预

读懂“字母面积图”

Price Ceiling 价格上限

Price Floor 价格下限

Per-Unit (Excise) Tax 从量税

Tax Incidence 税负归宿

Per-Unit Subsidy 补贴

Quantity Controls (Quotas) 数量管制

练习 Practice 2.8

2.9International Trade & Public Policy 国际贸易与贸易政策

Tariffs & Quotas 关税与进口配额

练习 Practice 2.9

ΣSummary 单元总结与易错点

2.1 Demand

2.2 Supply

2.3 PED

2.4 PES

2.5 YED & XED

2.6 Equilibrium & Surplus

2.7 Changes

2.8 Government Intervention

2.9 Trade

✎Unit 2 Test 单元测验

FRQFree-Response Practice 自由问答