📚 Year 9 Cambridge Economics Formula & Theorem Quick Reference Handbook | Year 9 剑桥经济公式定理速查手册
Economics is the study of how individuals, businesses and governments make choices to allocate scarce resources. This quick reference handbook summarises key formulas, laws and concepts you will encounter in Year 9 Cambridge Economics. Understanding these fundamentals will help you analyse real-world economic decisions and build a strong foundation for IGCSE Economics.
经济学研究个人、企业和政府如何做出选择以分配稀缺资源。本速查手册总结了你在 Year 9 剑桥经济课程中会遇到的关键公式、定律和概念。理解这些基础知识将帮助你分析现实世界的经济决策，并为 IGCSE 经济学打下坚实的基础。
1. Scarcity and the Basic Economic Problem | 稀缺性与基本经济问题
The basic economic problem is that resources are limited but human wants are unlimited. This forces us to make choices about what to produce, how to produce it, and for whom to produce it.
基本经济问题是资源有限而人类的需求无限。这迫使我们做出选择：生产什么、如何生产以及为谁生产。
Scarcity applies to all factors of production – land, labour, capital and enterprise. Even wealthy societies face scarcity because wants always exceed the means to satisfy them.
稀缺性适用于所有生产要素——土地、劳动力、资本和企业家才能。即使富裕社会也面临稀缺性，因为需求总是超过满足它们的手段。
2. Opportunity Cost Formula | 机会成本公式
Opportunity cost is the value of the next best alternative that is given up when a choice is made. It is the central concept of economic decision-making.
机会成本是做出选择时所放弃的次优替代方案的价值。它是经济决策的核心概念。
Opportunity Cost = Value of the Next Best Alternative Forgone
For example, if a student decides to spend an hour studying economics instead of playing video games, the opportunity cost is the enjoyment (or other benefit) they would have received from gaming.
例如，如果一名学生决定花一小时学习经济学而不是玩电子游戏，那么机会成本就是他们本可以从游戏中获得的乐趣（或其他收益）。
The opportunity cost of a government building a new hospital might be the schools or roads it could have built with the same money.
政府建造一所新医院的机会成本可能是用同样资金可以建造的学校或道路。
3. Factors of Production | 生产要素定理
Goods and services are produced using four categories of resources, known as factors of production: land, labour, capital and enterprise.
商品和服务使用四类资源生产，称为生产要素：土地、劳动力、资本和企业家才能。
Land includes all natural resources (oil, water, forests). Labour is the human effort (physical and mental). Capital is man-made goods used in production (machines, tools, factories). Enterprise is the ability to combine the other three factors to produce goods and take risks.
土地包括所有自然资源（石油、水、森林）。劳动力是人类付出的努力（体力和脑力）。资本是生产中使用的制成品（机器、工具、工厂）。企业家才能是将其他三种要素结合起来生产商品并承担风险的能力。
The reward (or income) for each factor: land earns rent, labour earns wages, capital earns interest, and enterprise earns profit.
每种要素的回报（或收入）：土地获得地租，劳动力获得工资，资本获得利息，企业家才能获得利润。
4. Demand and the Law of Demand | 需求与需求定律
Demand is the quantity of a good or service that consumers are willing and able to buy at various prices over a given period of time, ceteris paribus (all other things being equal).
需求是指在特定时期内，在其他条件不变的情况下，消费者愿意并能够在不同价格下购买的商品或服务的数量。
Law of Demand: Price ↑ → Quantity Demanded ↓ (and vice versa), ceteris paribus
There is an inverse relationship between price and quantity demanded. When the price of a product rises, consumers tend to buy less, either because they switch to cheaper substitutes or because their real income effectively falls (income effect).
价格与需求量之间存在反向关系。当产品价格上升时，消费者倾向于购买更少，他们或转向更便宜的替代品，或因为实际收入有效下降（收入效应）。
Movements along the demand curve are caused by a change in the good's own price; shifts of the demand curve are caused by non-price factors such as changes in income, tastes, population or the price of related goods (substitutes and complements).
需求曲线上的移动是由该商品本身价格变化引起的；需求曲线移动是由非价格因素引起的，例如收入变化、偏好、人口或相关商品（替代品和互补品）价格的变化。
5. Supply and the Law of Supply | 供给与供给定律
Supply is the quantity of a good or service that producers are willing and able to offer for sale at various prices over a given period of time, ceteris paribus.
供给是指在特定时期内，在其他条件不变的情况下，生产者愿意并能够在不同价格下提供销售的商品或服务的数量。
Law of Supply: Price ↑ → Quantity Supplied ↑ (and vice versa), ceteris paribus
There is a direct (positive) relationship between price and quantity supplied. Higher prices give producers an incentive to increase output because they can earn higher profits and cover the rising marginal cost of additional production.
价格与供给量之间存在直接（正向）关系。更高的价格激励生产者增加产量，因为他们可以获得更高利润，并能覆盖额外生产带来的上升边际成本。
Shifts of the supply curve are due to factors like changes in costs of production, technology, taxes, subsidies or the number of sellers in the market.
供给曲线的移动由诸如生产成本变化、技术、税收、补贴或市场中卖家数量等因素引起。
6. Market Equilibrium | 市场均衡
Market equilibrium occurs where quantity demanded equals quantity supplied at a particular price. This is the 'clearing price' – there is no shortage or surplus.
市场均衡出现在需求量等于供给量的特定价格处。这就是'出清价格'——没有短缺或过剩。
Equilibrium condition: Qd = Qs
If the market price is above equilibrium, there is excess supply (surplus), which puts downward pressure on price. If the price is below equilibrium, there is excess demand (shortage), pushing price upward. These forces tend to move the market towards equilibrium.
如果市场价格高于均衡，则存在超额供给（过剩），对价格构成下行压力。如果价格低于均衡，则存在超额需求（短缺），推动价格上升。这些力量倾向于使市场走向均衡。
Changes in demand or supply cause shifts in the curves and change the equilibrium price and quantity. For example, an increase in demand (shift right) raises both price and quantity.
需求或供给的变化导致曲线移动，并改变均衡价格和数量。例如，需求增加（右移）会提高价格和数量。
7. Price Elasticity of Demand (PED) Basics | 需求价格弹性基础
Price elasticity of demand measures how responsive the quantity demanded of a good is to a change in its price. It helps firms predict the effect of price changes on total revenue.
需求价格弹性衡量商品需求量对其自身价格变化的反应程度。它帮助企业预测价格变动对总收入的影响。
PED = (% Change in Quantity Demanded) / (% Change in Price)
PED = 1, it is unitary elastic. If PED = 0, perfectly inelastic; if infinite, perfectly elastic.
如果 PED > 1，需求有弹性（消费者对价格变化敏感）。如果 PED < 1，需求缺乏弹性（消费者相对不敏感）。如果 PED = 1，为单位弹性。如果 PED = 0，为完全无弹性；如果无穷大，为完全弹性。
Factors affecting PED include the availability of close substitutes, whether the good is a necessity or luxury, the proportion of income spent on the good, and the time period considered.
影响 PED 的因素包括：相近替代品的可获得性、该商品是必需品还是奢侈品、花费在该商品上的收入比重以及所考虑的时间段。
8. Costs of Production | 生产成本
Firms incur costs when producing goods and services. Understanding cost structures is essential for analysing profit maximisation.
企业在生产商品和服务时会产生成本。了解成本结构对于分析利润最大化至关重要。
Total Cost (TC) = Total Fixed Cost (TFC) + Total Variable Cost (TVC)
Fixed costs do not vary with output in the short run (e.g. rent, salaries). Variable costs change directly with the level of production (e.g. raw materials, energy).
固定成本在短期内不随产量变化（如租金、工资）。可变成本直接随生产水平变化（如原材料、能源）。
Average Cost (AC or ATC) = Total Cost / Quantity (Q)
Marginal Cost (MC) = Change in Total Cost / Change in Quantity (ΔTC/ΔQ)
Marginal cost is the extra cost of producing one more unit. When MC < AC, AC is falling; when MC > AC, AC rises. MC cuts AC at its lowest point.
边际成本是增加一单位生产所带来的额外成本。当 MC < AC 时，AC 下降；当 MC > AC 时，AC 上升。MC 在 AC 的最低点与 AC 相交。
9. Revenue and Profit Formulas | 收益与利润公式
Revenue is the income a firm receives from selling its output. Profit is the financial reward for enterprise and the motive for production in a market economy.
收益是企业从销售其产量中获得的收入。利润是企业家才能的经济奖励，也是市场经济中进行生产的动机。
Total Revenue (TR) = Price (P) × Quantity Sold (Q)
Profit (π) = Total Revenue (TR) – Total Cost (TC)
If TR > TC, the firm makes a profit. If TR < TC, it makes a loss. Normal profit is the minimum profit needed to keep a firm in its current line of business; it is included in total cost (in economics). Supernormal profit (or abnormal profit) is any profit above normal profit.
如果 TR > TC，企业盈利。如果 TR < TC
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