📚 Year 8 WJEC Economics: Quick Reference to Formulas and Theorems | 8年级 WJEC 经济学：公式定理速查手册
This quick reference guide covers the essential formulas, definitions, and economic theorems from the Year 8 WJEC Economics curriculum. Each section presents a core concept with a concise English explanation followed by its matched Chinese translation. Use it to revise topics such as scarcity, opportunity cost, supply and demand, elasticity, and market equilibrium.
本速查手册涵盖8年级 WJEC 经济学课程中的关键公式、定义和经济定理。每一节都先给出一段简明的英文解释，再附上对应的中文翻译。可供复习稀缺性、机会成本、供求关系、弹性和市场均衡等主题时使用。
1. Scarcity and Choice | 稀缺与选择
Scarcity means that there are finite resources but infinite human wants. Because of scarcity, every society must make choices about what to produce, how to produce, and for whom to produce.
稀缺性意味着资源是有限的，而人的欲望是无限的。由于稀缺性的存在，每个社会都必须做出选择，决定生产什么、如何生产以及为谁生产。
2. Opportunity Cost | 机会成本
Opportunity cost is the value of the next best alternative that is forgone when a choice is made. It is not always measured in money; it can be expressed in terms of time, enjoyment, or other benefits given up.
机会成本是做出某种选择时所放弃的次优选项的价值。它不一定用金钱衡量，也可以用所放弃的时间、享受或其他收益来表示。
Opportunity Cost = Value of the Next Best Alternative Forgone
3. Factors of Production | 生产要素
The four factors of production are land (natural resources), labour (human effort), capital (man-made tools and machinery), and enterprise (the willingness to take risks and organise the other three factors). Payments for these factors are rent, wages, interest, and profit respectively.
四种生产要素是土地（自然资源）、劳动力（人力劳动）、资本（人造的工具和机械）以及企业家才能（承担风险并组织其他三种要素的意愿）。它们各自的报酬是地租、工资、利息和利润。
4. Production Possibility Frontier | 生产可能性边界
A Production Possibility Frontier (PPF) shows the maximum combinations of two goods that an economy can produce with full employment of its existing resources and technology. Points on the curve represent efficiency; points inside represent underemployment; points outside are unattainable with current resources.
生产可能性边界（PPF）表示一个经济体在充分利用现有资源和技术时所能生产的两种商品的最大产量组合。边界上的点代表有效率的生产；边界内的点表示资源未充分利用；边界外的点用现有资源无法达到。
PPF Slope = Opportunity Cost of One Good in Terms of the Other
5. Demand | 需求
Demand refers to the quantity of a good or service that consumers are willing and able to buy at various prices in a given period. The Law of Demand states that, ceteris paribus, as the price rises, quantity demanded falls, and vice versa.
需求是指消费者在不同价格水平下愿意并且能够购买的商品或服务的数量。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；价格下降，需求量上升。
Demand Function: Qd = f(P, Y, T, Ps, N, ...)
good, Y = income, T = tastes, Ps = price of substitutes/complements, N = number of buyers.
其中 Qd = 需求量，P = 商品价格，Y = 收入，T = 偏好，Ps = 替代品/互补品价格，N = 购买者数量。
6. Supply | 供给
Supply is the quantity of a good or service that producers are willing and able to offer for sale at various prices in a given period. The Law of Supply states that, ceteris paribus, as the price rises, quantity supplied increases, and vice versa.
供给是指生产者在不同价格水平下愿意并且能够提供销售的商品或服务的数量。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；价格下降，供给量减少。
Supply Function: Qs = f(P, C, Tech, G, ...)
supplied, P = price, C = costs of production, Tech = technology, G = government policies.
其中 Qs = 供给量，P = 价格，C = 生产成本，Tech = 技术，G = 政府政策。
7. Market Equilibrium | 市场均衡
Market equilibrium occurs where the quantity demanded equals the quantity supplied. The equilibrium price (P*) clears the market, meaning there is no shortage or surplus. If price is above equilibrium, a surplus exists; if below, a shortage occurs.
市场均衡发生在需求量等于供给量时。均衡价格（P*）使市场出清，即不存在短缺或过剩。若价格高于均衡价格，则出现过剩；若低于均衡价格，则出现短缺。
At Equilibrium: Qd = Qs
8. 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. PED is usually negative, but we often ignore the sign.
需求价格弹性（PED）衡量需求量对价格变化的反应程度。计算方法为需求量变化的百分比除以价格变化的百分比。PED 通常为负值，但我们常忽略其符号。
PED = %ΔQd / %ΔP
| PED Value | Interpretation | 解释 |
| |PED| > 1 | Elastic demand | 富有弹性 |
| |PED| < 1 | Inelastic demand | 缺乏弹性 |
| |PED| = 1 | Unit elastic demand | 单位弹性 |
9. Price Elasticity of Supply | 供给价格弹性
Price elasticity of supply (PES) measures how responsive quantity supplied is 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化的反应程度。它等于供给量变化的百分比除以价格变化的百分比。
PES = %ΔQs / %ΔP
(<1), or unit elastic (=1). Factors such as production time and spare capacity affect PES.
PES 可大于1（富有弹性）、小于1（缺乏弹性）或等于1（单位弹性）。生产时间、闲置产能等因素会影响 PES。
10. Costs, Revenue and Profit | 成本、收入与利润
Total revenue (TR) is the money a firm receives from selling its output. Total cost (TC) is the sum of all fixed and variable costs. Profit is the difference between total revenue and total cost.
总收入（TR）是企业出售产品所获得的货币总额。总成本（TC）是所有固定成本和可变成本之和。利润是总收入与总成本之间的差额。
TR = P × Q
Profit = TR – TC
Average cost (AC) is TC ÷ Q. When TR = TC, the firm breaks even.
平均成本（AC）= TC ÷ Q。当 TR = TC 时，企业达到盈亏平衡。
11. Economic Systems | 经济制度
Economic systems determine how resources are allocated. The main types are market economies (decisions made by consumers and firms), command economies (government makes all decisions), and mixed economies (a blend of market and government). WJEC focuses on how each system answers the basic economic questions.
经济制度决定资源的分配方式。主要类型有市场经济（由消费者和企业做决策）、计划经济（政府做所有决策）和混合经济（市场与政府相结合）。WJEC 侧重于每种制度如何回答基本经济问题。
12. Market Failure | 市场失灵
Market failure occurs when the free market fails to allocate resources efficiently. Common causes include externalities (spillover effects on third parties), public goods (non-excludable and non-rival), and information asymmetry. Governments may intervene with taxes, subsidies, or regulations to correct the failure.
市场失灵是指自由市场未能有效配置资源。常见原因包括外部性（对第三方的溢出效应）、公共物品（非排他性和非竞争性）以及信息不对称。政府可能通过税收、补贴或法规进行干预来纠正市场失灵。
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