# gyro永不抽风

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# Production Possibility Frontier (PPF) - 生产可能性边界

• PPF 向外凸起
• 线上：Efficient，线外：Impossible，线内：Inefficient

# Circular-Flow Diagram - 循环流向图

## Leakages - 漏出/流出

• Savings - 储蓄（人们把一部分钱存起来，市场上钱少了）
• Taxation - 税收（市场上的钱给了政府，钱少了）
• Imports - 进口（花钱买别人的东西，钱少了）

## Leakages - 漏出/流出

• Investment - 投资（投资者的钱进入市场，钱多了）
• Government Spending - 市政开销（政府花钱，钱流入市场，钱多了）
• Exports - 出口（东西卖给别人，钱多了）

# Demand & Supply - 需求和供给

## Demand和Quantity of Demand

• Demand是指整个需求 ———— 整个曲线
• Quantity of Demand是指需求量 ———— 曲线上的点

# 商品分类

## Normal Good & Inferior Good - 正常商品 / 低档商品

• Normal Good ： 人们有钱以后想买的东西
• Inferior Good ： 人们有钱以后不想买的东西

## Substitutes & Complements - 替代品 / 互补品

• Substitutes ： 一个东西卖得好，另外一个东西卖的差
• Complements ： 一个东西卖得好，另外一个东西也随之卖得好

# Control on Prices - 价格管制

## Taxation - 税收

Definition: Fluctuations in economic activity, such as employ- ment and production.

# Elasticity - 弹性

## 需求价格弹性

### 影响因素

1. 相近商品的可得性 Availability of Cloase Substitues: 具有相近替代品的商品，一般需求弹性较大
通俗理解：如果有替代品，你涨价，我就去买别人，所以这个对价格的变化需求量就会变化很大，所以弹性较大
2. 必需品和奢侈品 Necessities versus Luxuries: 必需品(necessities)通常而言缺乏弹性，奢侈品(luxuries)通常富有弹性
理解方法：刚需的弹性为零。奢侈品在涨价后，既然是奢侈品，那么可买可不买，所以奢侈品对价格的变动其需求量变化较大，弹性较大
3. 市场的定义 Definition of the Market: 狭义市场比广义市场更有需求弹性
理解方法：举个例子，如果要买纸巾：单一品牌纸巾的市场，相比于所有纸巾所构成的市场，弹性较大，因为整个市场需求量差不多是固定的，而一个品牌如果涨价，消费者就回去寻找替代品，这个和第一条规则类似
4. 时间维度 Time Horizon: 某商品的长期需求通常比短期需求更加具有弹性
理解方法：我加钱可以一点一点的涨，到头来还是要买

### 衡量

• $\varepsilon > 1$ : Elastic
• $\varepsilon = 1$ : Unit Elasticity
• $\varepsilon < 1$ : Inelastic
• $\varepsilon \rightarrow \infty$ : Perfectly Elastic
• $\varepsilon = 0$ : Perfectly Inelastic

## 需求收入弹性

• 正常商品(Normal Goods) : $\varepsilon > 0$ （可理解为需求随工资的增高而增高）
• 低档商品(Inferior Goods) : $\varepsilon < 0$ （可理解为需求随工资的增高而降低）

## 需求的交叉价格弹性

• 替代品(Subtitutes) : $\varepsilon > 0$ （可理解为，一个东西需求增高，另一个东西价格也升高，意味着其需求降低，那么两者就位替代品）
• 互补品(Complements) : $\varepsilon < 0$ （可理解为，一个东西需求增高，另一个东西价格降低，意味着其需求增高，那么两者就位互补品）

## 供给弹性

### 影响的Factors

1. 卖方变动自己产量的灵活性
举个例子：买海滨别墅的inelastic，卖书的elastic
2. 时间维度：长期弹性更大
和需求价格弹性相同

# Welfare Economics - 福利经济学

## 重要概念

1. Consumer Surplus = Value to buyers – Amount paid by buyers = 需求曲线之下，价格线之下的面积
2. Producer Surplus = Amount received by sellers – Cost to sellers = 供给曲线之上，价格线之下的面积
3. Total Surplus = CS + PS = Willingness To Pay - Cost，体现市场效率
4. 边际买者：价格提高后，第一个离开市场的人
5. 边际卖者：价格下降后，第一个离开市场的人

## 要点

1. 开放国际贸易，就要接受国际的价格
2. 如果World Price > Domestic Price，出口
3. 如果World Price < Domestic Price，进口
4. Tariff有无谓损失，Quota没有
5. Tariff政府拿得到钱，有Tax Revenue，Quota没有

## 国际贸易的好处

1. 商品多样性增加
2. 规模经济降低成本（economies of scale）
3. 竞争程度增加
4. 科技流动性增加

# 企业行为

## Profit Maximizing - 利润最大化

Profit Maximizing - 发生在 MC = MR

## Economies of Scale - 规模经济

• $\text {Q} \uparrow \rightarrow \text {ATC} \downarrow$ 称为 economies of scale - 规模经济
长期ATC下降
• $\text {Q} \uparrow \rightarrow \text {ATC} 不变$ 称为 constant return to scale - 规模报酬不变
长期ATC不变
• $\text {Q} \uparrow \rightarrow \text {ATC} \uparrow$ 称为 diseconomies of scale - 规模不经济
长期ATC上升

## returns to scale - 规模报酬

$\delta > \alpha$ 产出扩大的倍数更多，increasing return to scale
$\delta = \alpha$ 产出扩大的倍数和投入相等，constant return to scale
$\delta < \alpha$ 产出扩大的倍数少，decreasing return to scale

## 短期和长期

### 短期成本和长期成本

$\Longrightarrow$所有短期ATC曲线都位于长期ATC曲线之上或上方

### 长期经营的一个重要事实

All factors of production are variable

# Perfectly Competitive Market - 完全竞争市场

## 完全竞争市场的特征

1. 无限多的买者和卖者 - Infinite buyers & sells
2. 所有卖的商品都一样 - All the goods are identical
3. 可以自由进入或退出 - Firms can freely exit or enter the market

## 完全竞争市场的特点

$AR = MR = P$

# Monopoly - 完全垄断市场

## 完全垄断市场特征

A firm that is the sole seller of a product without close substitutes

1. 唯一的卖者 - Sole seller of a product
2. 没有替代品 - Without Close Substitute

## 完全垄断市场的特点（在没有价格歧视的情况下）

$MC=MR$永远是Profit Maximizing的方法（根据边际产量递减效应）

$MR$和$D$的曲线具有两倍斜率关系

## Price Discrimination - 价格歧视

2. 别人不会转手卖掉（resale）

## 垄断的公共政策

1. 反托拉斯法案（Antitrust Law）：组织公司间的合并，允许政府拆分公司。增加市场的竞争性
2. 对垄断行为管制
3. 将私人垄断企业变成公共事业
4. 什么也不做（药品专利）

# Monopolistic Competition - 垄断竞争

## 垄断竞争的特征

1. 很多的卖者：很多企业竞争相同的客户群体 - Many Sellers
2. 产品差异化：每个企业生产的和别人的都略有不同，需求曲线向下倾斜 - Product Differentiation
3. 自由进出市场 - Free entry and exit

## 垄断竞争的特点

Advertise的原因：因为大家的产品比较相似，所以打广告就是为了让别人了解到你的产品，想让消费者选择这个产品，让价格弹性变小。yy：$\varepsilon = 0$时是刚需

# Oligopoly - 寡头垄断

## Nash Equilibrium - 纳什均衡

Prisoners’ Dilemma - 囚徒困境

## 一道恶心题

• Efficiency : 既可以指Allocation，也可以指Productive
• 在这里两个公司合并，ATC必然下降（因为享受了economies of scales, AFC减少）

# 生产要素市场 - The Markets for the Factors of Production

## Input / Factors of Production - 生产要素

• Labor - 劳动力
• Human Capital - 人力资本
• Natural Resources / Land - 自然资源
• Capital (Physical Capital) - 实物资本

## 曲线的移动

### 劳动需求曲线的移动

1. 产出物的价格：$P \uparrow \longrightarrow VMPL \uparrow \longrightarrow D \uparrow$
2. 技术变革
• 劳动节约型
• 历史角度：劳动增加型
3. 其他生产要素的原因

### 劳动供给曲线的移动

1. 爱好改变 e.g. 女性参加工作
2. 其他工作机会的改变
3. 移民

# Externality - 外部性

• Positive Externality: 做一件事对别人有好处
• Negative Externality: 做一件事对别人有坏处

## 针对外部性的政策

1. 命令和控制政策（Command-and-Control Policies）：直接管制行为
2. 基于市场的政策（Market-Based Policies）：激励私人解决（更有效率）
• Corrective Taxes（矫正税） / Pigovian Taxes（庇古税）

# 物品的性质和种类

## 物品的性质

• 排他性（Excludability） ：是否能阻止别人用这个东西：你用了别人不能用
• 竞争性（Rivalry in consumption） ：别人用了，就不值钱了

## 物品的种类

• 私人物品（Private Goods） ：有排他性、有竞争性
• e.g. 裙子 / 拥挤的收费公路
• 自然垄断（Natural Monopoly） ：有排他性、无竞争性
• e.g. 消防 / 不拥挤的收费道路
• 公共资源（Common Resources） ：无排他性、有竞争性
• e.g. 海鱼 / 拥挤的不收费道路
• 公地悲剧（由负外部性造成）：土地是大家的，都放羊，放多了，土地坏掉了，大家都没得放。
• 公共物品（Public Goods) ：无排他性、无竞争性
• e.g. 国防 / 不拥挤的不收费道路
• Free Rider Problem，最后有讲。

# 收入不平等

## 穷困率 - Poverty Rate

• 穷困率：家庭收入低于穷困线的人百分比
• 穷困线：政府制定的标准，低于这个水平就属于生活在贫困之中

## 洛伦兹曲线和基尼系数 Lorenz Curve & Gini Coefficient

• 曲线越靠里，越极端，收入越不平等
• 曲线越接近 y = x（基准线），收入越平等

# 消费者选择理论

## 无差异曲线 - Indifference Curve

• Property 1: Higher indifference curves are preferred to lower ones. People usually prefer to consume more goods rather than less. This preference for greater quantities is reflected in the indifference curves. As Figure 2 shows, higher indifference curves represent larger quantities of goods than lower indiffer- ence curves. Thus, the consumer prefers being on higher indifference curves.

• Property 2: Indifference curves are downward sloping. The slope of an indiffer- ence curve reflects the rate at which the consumer is willing to substitute one good for the other. In most cases, the consumer likes both goods. Therefore, if the quantity of one good is reduced, the quantity of the other good must increase for the consumer to be equally happy. For this reason, most indiffer- ence curves slope downward.

• Property 3: Indifference curves do not cross. To see why this is true, suppose that two indifference curves did cross, as in Figure 3. Then, because point A is on the same indifference curve as point B, the two points would make the consumer equally happy. In addition, because point B is on the same indif- ference curve as point C, these two points would make the consumer equallyhappy. But these conclusions imply that points A and C would also make the consumer equally happy, even though point C has more of both goods. This contradicts our assumption that the consumer always prefers more of both goods to less. Thus, indifference curves cannot cross.

• Property 4: Indifference curves are bowed inward. The slope of an indifference curve is the marginal rate of substitution—the rate at which the consumer is willing to trade off one good for the other. The marginal rate of substitu- tion (MRS) usually depends on the amount of each good the consumer is currently consuming. In particular, because people are more willing to trade away goods that they have in abundance and less willing to trade away goods of which they have little, the indifference curves are bowed inward. As an example, consider Figure 4. At point A, because the consumer has a lot of Pepsi and only a little pizza, he is very hungry but not very thirsty. To induce the consumer to give up 1 pizza, he has to be given 6 pints of Pepsi: The marginal rate of substitution is 6 pints per pizza. By contrast, at point B, the consumer has little Pepsi and a lot of pizza, so he is very thirsty but not very hungry. At this point, he would be willing to give up 1 pizza to get 1 pint of Pepsi: The marginal rate of substitution is 1 pint per pizza. Thus, the bowed shape of the indifference curve reflects the consumer’s greater willingness to give up a good that he already has in large quantity.

## Utility - 效用

• 可以理解为benefit。
• 边际效用 - Marginal Utility
• 重点： Diminishing Marginal Utility：消费者拥有某商品越多，在额外增加一单位，带来的快乐低。

# AP 2020 特辑

## 考试信息

1. 考试时间：北京时间5月21日凌晨4点
2. 两道FRQ
• Question 1 (55%): 25 minutes + 5 minutes to upload response
• Question 2
3. 删减一个单元
Unit 6 Market Failure and the role of Government (8% - 13%)
4. Q1 = FRQ2 + FRQ3
5. Q2 = FRQ1

## 考点

Units Exam Weighting
Unit 1: Basic Economic Concepts 12-15%
Unit 2: Supply and Demand 20-25%
Unit 3: Production, Cost, and the Perfect Competition model 22-25%
Unit 4: Imperfect Competition 15-22%
Unit 5: Factor Markets 10-13%

## 考察重点

• Unit1需要明确 definition
• Unit2几乎一定会融合在不同市场形式中考察(Unt3+4+5)
• 从完全竞争(side- by-side graph)/完全垄断/垄断竞争市场的图像中考察需求、供给曲线,弹性等
• Unit3-4一定会出1道题
• 市场之间的转化(垄断竞争完全竞争、寡头垄断-完全垄断)
• 每个市场的图像变化及结论
• Unit5有可能单独出题,其中一定会包含Unit2的知识点

## Unit 1 - FRQ - 1

Nirali is a student at the University of Ainsley. She has 5 hours to study for two exams today. The tables below show Nirali’s expected scores given the amount of time she studies for each exam.

Number of hours Spent Studying Microeconomics Expected Score on Microeconomics exam (100-point scale)
5 100
4 96
3 90
2 82
1 60
0 0
Number of hours Spent Studying History Expected Score on History exam (100-point scale)
0 0
1 40
2 60
3 72
4 77
5 80

(a) Nirali spends 3 hours studying microeconomics and 2 hours studying history. Calculate her gain from the second hour spent studying history.

Solution: $40 \rightarrow 60$, $20$ points on History Exam.

(b) Calculate Nirali’s opportunity cost of the second hour spent studying history.

Solution: $6$ points on Microeconomics Exam.

(c) Assume Nirali increases the time she allocates to studying history. What happens to the opportunity cost of studying history? Explain

Solution: The opportunity cost of studying history will increase.
Explanation:

• Method 1: Expected score on Microeconmics decreases at an increasing rate for each hour spent on histroy.
• Method 2: Marginal cost is increasing.

(d) Assume that nirali has a goal of maximizing the sum of her test scores( the score on microeconomics plus the score on history). How many hours should she study for each exam?

Solution: She should study Microeconomics for $2$ hours and study Histroy for $3$ hours, which will enable her to achieve a total score of $154$ points, the maximum total score she can get.

(e) Nirali learns that her tennis practice has been canceled, freeing up an additional hour for studying. Given your answer to part(d), will Nirali allocate the additional hour to studying microeconomics or to studying history to maximize the sum of her test scores? Explain using marginal analysis

Solution: She will allocate the additional hour to studying microeconomics.
Explanation: Because the marginal benefit of learning an additional hour of microeconomics is $90 - 82 = 8$ points and the marginal benefit of learning an additional hour of history is $77 - 72 = 5$. $8 > 5$.

## Unit 1 - FRQ - 2

Sasha is a utility-maximizing consumer who spends all of her income on peanuts and bananas, both of which are normal goods

• (a) Assume that the last unit of peanuts consumed increased Sasha’s total utility from 40 utils to 48 utils and that the last unit of bananas consumed increased her total utility from 52 utils to 56 utils
• (i) If the price of a unit of peanuts is SI and Sasha is maximizing utility, calculate the price of a unit of bananas
• Solution: Thus the price of banana is $(56 - 52) \div (48 - 40) \times 1 = 0.5$ dollars
• (ii) If the price of a unit of peanuts increases and the price of a unit of bananas remains unchanged from the price you determined in part(a)(i), how will Sasha’s purchase of peanuts change?
• Solution: She will purchase less peanuts.
• (b) Assume that the cross-price elasticity of demand between peanuts and bananas is positive. A widespread disease has destroyed the banana crop. What will happen to the equilibrium price and quantity of peanuts in the short run? Explain
• (c) Assume that the price of bananas increases.
• (i) Will the substitution effect increase, decrease, or have no effect on the quantity of bananas demanded?
• (ii) What happens to Sasha’s real income?

## Unit 2 - FRQ - 1

The graph below shows the market for widgets. The government is considering intervening in this market.

• (a) Calculate the total producer surplus at the market equilibrium price and quantity. Show your work.
• Solution: $20 \times 20 \div 2 = 200$ dollars
• (b) If the government imposes a price floor at $16, is there a shortage, a surplus, or neither? Explain • Solution: Neither, because the price floor is below the equilibrium, thus it is not binding / not effective / ineffective. • (c) If instead the government imposes a price ceiling at$12, is there a shortage, a surplus, or neither? Explain
• Solution: There will be a shortage, because the price ceiling is below the equilibrium and $Q_D > Q_S$ (Quantity Demanded is greater than Quantity Supplied).
• (d) If instead the government restricts the market output to 10 units, calculate the deadweight loss. Show your work.
• Solution: $(40 - 10) \times (20 - 10) \div 2 = 150$ dollars.
• (e) Assume the price decreases from 20 dollars to 12 dollars.
• (i) Calculate the price elasticity of demand. Show your work.
• Solution:
• (ii) In this price range, is demand perfectly elastic, relatively elastic, unit elastic, relatively inelastic, or perfectly inelastic?
• Solution: Since $\varepsilon < 1$, the demand is relatively inelastic in this price range.

## Unit 2 - FRQ - 2

The graph above illustrates the market for calculators. S denotes the current supply curve, and D denotes the demand curve.

• (a) Calculate the producer surplus before the tax
• Solution: $(5 - 2) \times 90 \div 2 = 135$ dollars.

## Unit 5 - FRQ - 2 - Monopsony（单一Employer）

Woodland is a small town in which everyone works for TreeMart, the local lumber company. TreeMart is a monopsonist in the labor market and a perfect competitor in the lumber market. In the short run, labor is the only variable input. The labor market for TreeMart is given in the graph above

• (a) Identify the profit-maximizing quantity of labor for TreeMart
• Solution: $MC = MR$, 100.
• (b)Identify the wage rate Tree Mart pays to hire the profit-maximizing quantity of labor
• Solution: $10 • (c) Identify the quantity of labor hired in each of the following situations. • (i) Tree Mart operates in a competitive labor market • Solution: Competitive$\rightarrow S = D$, 200. • (ii) The government imposes a minimum wage of$12.5 Explain.
• Solution: With the minimum wage of \$12.5, according to the supply curve of labor, the company will hire 150 units of labor.

## 重要注意点

1. 写清楚单位
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