📚 Year 11 WJEC Economics: Quick-Reference Handbook of Formulas and Theorems | WJEC 11年级经济学：公式定理速查手册
This handbook compiles every essential formula, theorem and quantitative rule you need for the WJEC GCSE Economics examination. Keep it close for last-minute revision and practice calculations.
本手册汇集了 WJEC GCSE 经济学考试所需的全部核心公式、定理和定量法则，适合考前快速复习和计算练习。
1. Scarcity, Choice and Opportunity Cost | 稀缺性、选择与机会成本
The fundamental economic problem arises because resources are finite but human wants are infinite. Scarcity forces individuals, firms and governments to make choices.
基本经济问题源于资源有限而人类欲望无限。稀缺性迫使个人、企业和政府做出选择。
Opportunity cost is defined as the value of the next best alternative forgone when a choice is made. It is not necessarily monetary; it can be time, enjoyment or any other benefit.
机会成本是指做出某种选择时所放弃的次优选项的价值。它不一定是货币成本，可以是时间、乐趣或其他任何被牺牲的收益。
When a country moves from producing 100 units of cloth to 120 units of wheat on its Production Possibility Frontier (PPF), the opportunity cost of the extra 20 wheat is the cloth output lost.
当一个国家在生产可能性边界上从生产100单位布改为生产120单位小麦时，额外20单位小麦的机会成本就是所放弃的布匹产量。
Opportunity Cost = What you give up / What you gain
机会成本 = 放弃的价值 / 获得的价值
This ratio helps compare trade-offs. A straight-line PPF indicates constant opportunity cost, while a concave PPF implies increasing opportunity cost.
这一比率有助于比较取舍。直线的生产可能性边界表示机会成本不变，而凹向原点的边界意味着机会成本递增。
2. Demand and Supply | 需求与供给
The law of demand states that, ceteris paribus, as the price of a good rises, the quantity demanded falls. The law of supply states that as price rises, quantity supplied rises.
需求定理指出，在其他条件不变的情况下，商品价格上升，需求量下降。供给定理指出，价格上升，供给量上升。
In linear analysis, demand and supply can be written as:
在线性分析中，需求与供给可表示为：
Demand: Qd = a − bP
需求： Qd = a − bP
Supply: Qs = c + dP
供给： Qs = c + dP
Market equilibrium occurs where Qd = Qs. Solve for P* and Q*.
市场均衡出现在 Qd = Qs 时。求解可得均衡价格 P* 和均衡数量 Q*。
A shift in demand is caused by non-price factors such as income, tastes, price of substitutes/complements; a shift in supply is caused by changes in costs, technology, taxes or subsidies.
需求的变动由非价格因素引起，如收入、偏好、替代品/互补品价格；供给的变动则由成本、技术、税收或补贴等因素引起。
3. Elasticities | 弹性
Elasticity measures the responsiveness of one variable to a change in another. The four key elasticities at GCSE are PED, PES, YED and XED.
弹性衡量一个变量对另一个变量变化的反应程度。GCSE 阶段需要掌握四种弹性：PED、PES、YED 和 XED。
Price Elasticity of Demand (PED) = %Δ Qd ÷ %Δ P
需求价格弹性 (PED) = 需求量变动% ÷ 价格变动%
Using the midpoint formula to avoid inconsistency:
使用中点公式可避免基期选择带来的不一致：
PED = (ΔQ ÷ ((Q₁+Q₂)/2)) ÷ (ΔP ÷ ((P₁+P₂)/2))
PED = (ΔQ ÷ ((Q₁+Q₂)/2)) ÷ (ΔP ÷ ((P₁+P₂)/2))
| PED Value | Type | 中文解释 |
|---|---|---|
| > 1 | Elastic | 富有弹性 |
| = 1 | Unit elastic | 单位弹性 |
| < 1 | Inelastic | 缺乏弹性 |
| = 0 | Perfectly inelastic | 完全无弹性 |
| = ∞ | Perfectly elastic | 完全弹性 |
Price Elasticity of Supply (PES) = %Δ Qs ÷ %Δ P
供给价格弹性 (PES) = 供给量变动% ÷ 价格变动%
Income Elasticity of Demand (YED) = %Δ Qd ÷ %Δ Income
需求收入弹性 (YED) = 需求量变动% ÷ 收入变动%
Normal goods have a positive YED; inferior goods have a negative YED. Luxury goods typically show YED > 1.
正常品 YED 为正；低档品 YED 为负。奢侈品通常 YED > 1。
Cross Elasticity of Demand (XED) = %Δ Qd of good A ÷ %Δ P of good B
需求交叉弹性 (XED) = 商品A需求量变动% ÷ 商品B价格变动%
Substitutes have positive XED; complements have negative XED.
替代品 XED 为正，互补品 XED 为负。
4. Consumer Surplus and Producer Surplus | 消费者剩余与生产者剩余
Consumer surplus is the difference between what consumers are willing to pay and the market price they actually pay, summed over all units purchased.
消费者剩余是消费者愿意支付的最高价格与实际支付的市场价格之间的差额，对所有购买单位求和。
Consumer Surplus = ½ × (Max price − Market price) × Equilibrium quantity
消费者剩余 = ½ ×（最高支付意愿价格 − 市场价格）× 均衡数量
Producer surplus is the difference between the market price and the minimum price at which producers are willing to supply.
生产者剩余是市场价格与生产者愿意接受的最低价格之间的差额。
Producer Surplus = ½ × (Market price − Min supply price) × Equilibrium quantity
生产者剩余 = ½ ×（市场价格 − 最低供给价格）× 均衡数量
Total welfare = Consumer surplus + Producer surplus. Taxes and subsidies cause shifts that change the surplus areas and may create deadweight loss.
总福利 = 消费者剩余 + 生产者剩余。税收与补贴会导致剩余区域变化，并可能产生无谓损失。
5. Costs of Production | 生产成本
Firms face both fixed costs (independent of output) and variable costs (change with output).
企业既有固定成本（不随产量改变）也有可变成本（随产量改变）。
Total Cost (TC) = Total Fixed Cost (TFC) + Total Variable Cost (TVC)
总成本 (TC) = 总固定成本 (TFC) + 总可变成本 (TVC)
Average Cost (ATC) = TC ÷ Q
平均成本 (ATC) = 总成本 ÷ 产量 Q
Average Fixed Cost (AFC) = TFC ÷ Q
平均固定成本 (AFC) = 总固定成本 ÷ Q
Average Variable Cost (AVC) = TVC ÷ Q
平均可变成本 (AVC) = 总可变成本 ÷ Q
Marginal Cost (MC) = Δ TC ÷ Δ Q
边际成本 (MC) = 总成本变动量 ÷ 产量变动量
MC cuts AVC and ATC at their minimum points. When MC < ATC, ATC is falling; when MC > ATC, ATC is rising.
MC 曲线穿过 AVC 和 ATC 的最低点。当 MC 低于 ATC 时，ATC 下降；当 MC 高于 ATC 时，ATC 上升。
6. Revenue, Profit and Break-even | 收入、利润与盈亏平衡
Revenue is the money a firm receives from sales. Profit is the difference between total revenue and total cost.
收入是企业通过销售获得的款项。利润是总收入与总成本之差。
Total Revenue (TR) = Price × Quantity (P × Q)
总收入 (TR) = 价格 × 数量 (P × Q)
Average Revenue (AR) = TR ÷ Q = P
平均收入 (AR) = 总收入 ÷ Q = 价格
Profit = TR − TC
利润 = 总收入 − 总成本
Break-even occurs when TR = TC or AR = ATC. The break-even output can be found using the contribution margin method.
盈亏平衡发生在 TR = TC 或 AR = ATC 时。盈亏平衡产量可用贡献毛利法计算。
Break-even output = TFC ÷ (Selling price per unit − AVC)
盈亏平衡产量 = 总固定成本 ÷（单位售价 − 平均可变成本）
Above this output, the firm makes a profit; below it, a loss. The margin of safety is actual output minus break-even output.
产量高于此水平则盈利，低于此水平则亏损。安全边际是实际产量与盈亏平衡产量之差。
7. Macroeconomic
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