📚 AP Microeconomics Key Concepts Review (Part 1) | AP 微观经济知识点梳理（一）
AP Microeconomics is built on foundational concepts that explain how individuals and firms make decisions under conditions of scarcity. This first part of the review series covers the core principles: scarcity, opportunity cost, production possibilities, comparative advantage, supply and demand, equilibrium, and elasticity. Mastering these ideas is essential for success in the course and exam.
AP 微观经济学建立在一系列基础概念之上，这些概念解释了个人和企业在稀缺条件下如何进行决策。本系列的第一部分梳理核心原理：稀缺性、机会成本、生产可能性、比较优势、供给与需求、均衡以及弹性。掌握这些概念是通过课程与考试的关键。
1. Scarcity and Choice | 稀缺性与选择
Scarcity is the fundamental economic problem: resources are limited while human wants are unlimited. Because of scarcity, every society and individual must make choices about how to allocate resources.
稀缺性是经济学的基本问题：资源是有限的，而人类的欲望是无限的。由于稀缺性的存在，每个社会和个人都必须就如何分配资源做出选择。
Scarcity forces trade-offs. In economics, a trade-off means that to obtain more of one thing, you must give up something else. This concept leads directly to the idea of opportunity cost.
稀缺性迫使人们进行权衡取舍。在经济学中，权衡取舍意味着要获得更多的一种东西，就必须放弃其他东西。这个概念直接引出了机会成本的概念。
- Scarcity ≠ shortage. Scarcity is permanent; a shortage is temporary.
- 稀缺性不等于短缺。稀缺性是永久的，而短缺是暂时的。
- All economic systems (market, command, mixed) must deal with scarcity.
- 所有经济体制（市场、指令、混合）都必须应对稀缺性问题。
2. Opportunity Cost | 机会成本
Opportunity cost is the value of the next best alternative that must be forgone when a choice is made. It is not the sum of all possible alternatives, but only the single best one given up.
机会成本是指做出一种选择时所必须放弃的次优替代品的价值。它不是所有可能替代品的总和，而只是被放弃的唯一最优替代品。
Economists view cost as opportunity cost, not merely monetary cost. For example, the cost of attending college includes not only tuition but also the forgone earnings from working full-time.
经济学家将成本视为机会成本，而不仅仅是货币成本。例如，上大学的成本不仅包括学费，还包括因全职工作而放弃的收入。
Opportunity Cost = Return on Best Foregone Option – Return on Chosen Option
机会成本 = 放弃的最佳选择收益 – 所选选择的收益
3. Production Possibilities Curve (PPC) | 生产可能性曲线
The Production Possibilities Curve (PPC) shows the maximum combinations of two goods an economy can produce with given resources and technology. It illustrates scarcity, trade-offs, and opportunity costs.
生产可能性曲线（PPC）展示了一个经济体在给定资源和技术条件下能够生产的两种商品的最大组合。它体现了稀缺性、权衡取舍和机会成本。
Points on the curve are efficient, points inside the curve represent unemployment or inefficiency, and points outside are unattainable with current resources.
曲线上的点是有效率的生产，曲线内部的点代表失业或低效率，曲线外部的点则在当前资源条件下无法实现。
- A straight-line PPC indicates constant opportunity cost.
- 直线型 PPC 表明机会成本不变。
- A concave (bowed-out) PPC indicates increasing opportunity cost.
- 凹向原点的 PPC 表明机会成本递增。
- Economic growth shifts the PPC outward.
- 经济增长会使 PPC 向外移动。
4. Comparative Advantage and Trade | 比较优势与贸易
A person or country has an absolute advantage in producing a good if it can produce more output per unit of input. Comparative advantage exists when the opportunity cost of producing a good is lower than for another producer.
如果一个人或国家每单位投入能生产更多产出，就拥有绝对优势。当生产一种商品的机会成本低于另一个生产者时，就存在比较优势。
Trade based on comparative advantage allows both parties to consume beyond their individual PPCs. Specialization increases total output and welfare.
基于比较优势的贸易使双方都能消费到各自 PPC 之外的产品。专业化生产能增加总产出和福利。
Terms of trade must lie between the opportunity costs of the two producers.
贸易条件必须介于两个生产者的机会成本之间。
Calculating comparative advantage involves comparing opportunity cost ratios expressed as “good X per unit of good Y”.
计算比较优势需要比较以“每单位 Y 商品可获得的 X 商品”表示的机会成本比率。
5. Supply and Demand | 供给与需求
The law of demand states that as price increases, quantity demanded decreases, ceteris paribus. The demand curve is downward sloping due to substitution and income effects.
需求定律指出，在其他条件不变的情况下，价格上升，需求量下降。由于替代效应和收入效应，需求曲线向右下方倾斜。
The law of supply states that as price increases, quantity supplied increases, ceteris paribus. Producers are willing to offer more as price rises because of the profit motive.
供给定律指出，在其他条件不变的情况下，价格上升，供给量增加。受利润动机驱动，价格越高，生产者愿意提供越多。
- Determinants of demand: tastes, income, prices of related goods, expectations, number of buyers.
- 需求的决定因素：偏好、收入、相关商品价格、预期、买者数量。
- Determinants of supply: input prices, technology, expectations, government policy, number of sellers.
- 供给的决定因素：投入品价格、技术、预期、政府政策、卖者数量。
- A change in price causes a movement along the curve; a change in a determinant shifts the curve.
- 价格变动导致沿曲线移动；决定因素变动导致曲线平移。
6. Market Equilibrium | 市场均衡
Equilibrium occurs where quantity demanded equals quantity supplied. At this price, there is no tendency for change. The market-clearing price balances the plans of buyers and sellers.
当需求量等于供给量时达到均衡。在此价格下，不存在变动的趋势。市场出清价格使买方和卖方的计划相协调。
A surplus exists when quantity supplied exceeds quantity demanded at a given price, putting downward pressure on price. A shortage exists when quantity demanded exceeds quantity supplied, pushing price upward.
当某一价格下供给量大于需求量时，存在过剩，会对价格产生下行压力。当需求量大于供给量时，存在短缺，会推动价格上升。
Analyzing changes in equilibrium involves identifying shifts in supply or demand, determining the direction of the shift, and comparing new equilibrium price and quantity with the original.
分析均衡变动需要判断供给或需求是否移动、确定移动方向，并比较新均衡价格和数量与原始水平。
Double shift: when both curves shift, either price or quantity will be indeterminate depending on magnitudes.
双移动：当两条曲线都移动时，价格或数量其中之一会因幅度不同而不确定。
7. Price Elasticity of Demand | 需求价格弹性
Price elasticity of demand (PED) measures the responsiveness of quantity demanded to a change in price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price.
需求价格弹性（PED）衡量需求量对价格变动的反应程度。其计算为需求量变动百分比除以价格变动百分比。
PED = %ΔQd / %ΔP
需求价格弹性 = 需求量变动百分比 / 价格变动百分比
- |PED| > 1: elastic demand (luxuries, many substitutes)
- |PED| > 1：富有弹性（奢侈品、替代品多）
- |PED| < 1: inelastic demand (necessities, few substitutes)
- |PED| < 1：缺乏弹性（必需品、替代品少）
- |PED| = 1: unit elastic; |PED| = 0: perfectly inelastic; |PED| → ∞: perfectly elastic
- |PED| = 1：单位弹性；|PED| = 0：完全无弹性；|PED| → ∞：完全弹性
- Total revenue test: if demand is elastic, a price increase reduces total revenue; if inelastic, a price increase raises total revenue.
- 总收益检验：如果需求富有弹性，提价会减少总收益；如果缺乏弹性，提价会增加总收益。
8. Price Elasticity of Supply | 供给价格弹性
Price elasticity of supply (PES) measures the responsiveness of quantity supplied to a change in price. It is the percentage change in quantity supplied divided by the percentage change in price.
供给价格弹性（PES）衡量供给量对价格变动的反应程度。其计算为供给量变动百分比除以价格变动百分比。
PES = %ΔQs / %ΔP
供给价格弹性 = 供给量变动百分比 / 价格变动百分比
Key determinants of PES include time horizon (supply is more elastic in the long run), availability of inputs, and storability of the good.
PES 的主要决定因素包括时间范围（长期供给弹性更大）、投入品的可得性以及商品的可储存性。
- PES > 1: elastic supply (producers can easily increase output)
- PES > 1：富有弹性（生产者可轻易增加产出）
- PES < 1: inelastic supply (output is difficult to change quickly)
- PES < 1：缺乏弹性（产出难以迅速改变）
- Perfectly inelastic supply: PES = 0, vertical supply curve (e.g., fixed seats in a stadium)
- 完全无弹性供给：PES = 0，供给曲线垂直（如体育场固定座位）
Understanding elasticity helps governments predict tax burdens and firms set pricing strategies. The more inelastic side of the market bears a larger share of a tax.
理解弹性有助于政府预测税收负担，并帮助企业制定定价策略。市场中弹性较小的一方承担更大份额的税收。
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