📚 Year 12 Edexcel Economics: Formula & Theorem Quick Reference Handbook | Year 12 Edexcel 经济：公式定理速查手册
This handbook compiles the essential formulas, theorems, and key relationships that every Year 12 Edexcel Economics student must know. Covering both microeconomic and macroeconomic foundations — from elasticity calculations and tax incidence to the multiplier and macroeconomic equilibrium — this guide helps you revise efficiently and apply concepts accurately in exams. Each section pairs an English explanation with a Chinese translation, ensuring clarity for bilingual learners.
本手册汇编了 Year 12 Edexcel 经济学学生必须掌握的各类重要公式、定理和关键关系。内容涵盖微观与宏观经济基础——从弹性计算、税收归宿到乘数效应和宏观经济均衡——帮助你高效复习并在考试中准确应用。每个小节均以英文解释搭配中文翻译，确保双语学习者透彻理解。
1. Price Elasticity of Demand (PED) | 需求的价格弹性
PED measures the responsiveness of quantity demanded to a change in price.
需求价格弹性衡量需求量对价格变动的反应程度。
PED = %ΔQd ÷ %ΔP
Where %ΔQd = (ΔQd / Qd₀) × 100 and %ΔP = (ΔP / P₀) × 100.
其中 %ΔQd = (需求量的变化 ÷ 初始需求量) × 100，%ΔP = (价格的变化 ÷ 初始价格) × 100。
PED is always negative due to the law of demand, but we often use the absolute value. |PED| > 1 is elastic, |PED| < 1 is inelastic, |PED| = 1 is unit elastic.
由于需求定律，PED 恒为负数，但通常使用绝对值。|PED| > 1 为富有弹性，|PED| < 1 为缺乏弹性，|PED| = 1 为单位弹性。
Total revenue test: When demand is elastic, a price cut raises total revenue; when inelastic, a price cut lowers total revenue.
总收入检验：需求富有弹性时，降价增加总收入；缺乏弹性时，降价减少总收入。
Determinants include availability of substitutes, necessity vs. luxury, proportion of income, and time period.
影响因素包括替代品可得性、必需品与奢侈品、占收入比重以及时间长短。
2. Income Elasticity of Demand (YED) | 需求的收入弹性
YED shows how quantity demanded responds to changes in consumer income.
收入弹性衡量需求量对消费者收入变动的反应程度。
YED = %ΔQd ÷ %ΔIncome
Normal goods have positive YED (>0): necessities have 0 < YED < 1, luxuries have YED > 1. Inferior goods have negative YED (<0).
正常品的 YED 为正（>0）：必需品的 YED 介于 0 和 1 之间，奢侈品的 YED > 1。低档品的 YED 为负（<0）。
Firms use YED to forecast demand during economic cycles — sales of luxury goods are more sensitive to income changes.
企业依据 YED 预测经济周期中的需求——奢侈品的销售对收入变化更敏感。
3. Cross Elasticity of Demand (XED) | 需求的交叉价格弹性
XED measures the change in quantity demanded of one good when the price of another good changes.
交叉弹性衡量一种商品需求量对另一种商品价格变动的反应程度。
XED = %ΔQd of good A ÷ %ΔP of good B
e to zero → unrelated goods. The larger the absolute value, the stronger the relationship.
XED 为正 → 替代品；为负 → 互补品；接近零 → 无关商品。绝对值越大，关系越强。
4. Price Elasticity of Supply (PES) | 供给的价格弹性
PES measures the responsiveness of quantity supplied to a price change.
供给弹性衡量供给量对价格变动的反应程度。
PES = %ΔQs ÷ %ΔP
e time period, availability of spare capacity, stock levels, and ease of entry into the market.
PES > 1 富有弹性；< 1 缺乏弹性；= 1 单位弹性。主要决定因素包括时间长短、闲置产能、库存水平以及市场进入难度。
5. Consumer 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 what they actually pay. Producer surplus is the difference between the price received and the minimum price at which producers are willing to sell.
消费者剩余是消费者愿意支付的最高价格与实际支付价格之间的差额。生产者剩余是实际售价与生产者愿意接受的最低价格之间的差额。
Graphically, consumer surplus is the area under the demand curve above the market price; producer surplus is the area above the supply curve below the market price.
在图形中，消费者剩余是需求曲线之下、市场价格之上的面积；生产者剩余是供给曲线之上、市场价格之下的面积。
Changes in market prices, taxes, subsidies, and price controls directly alter these surpluses and can cause deadweight loss.
市场价格变化、征税、补贴和价格管制会直接改变这些剩余，并可能导致无谓损失。
6. Tax Incidence and Elasticity | 税收归宿与弹性
The incidence of an indirect tax depends on the relative price elasticities of demand and supply.
间接税的归宿取决于需求和供给的相对价格弹性。
Key rule: The more inelastic side of the market bears a greater share of the tax burden.
关键规则：市场弹性越小的一方承担的税负比例越大。
If demand is perfectly inelastic, consumers pay the entire tax; if supply is perfectly inelastic, producers pay it all.
若需求完全无弹性，消费者承担全部税款；若供给完全无弹性，生产者承担全部税款。
× new quantity sold; the deadweight loss is the lost surplus not captured by tax revenue.
税收收入 = 单位税额 × 新的交易量；无谓损失为未被税收收入覆盖的剩余损失。
7. Subsidies and Government Intervention | 补贴与政府干预
A subsidy is a payment from the government to producers, lowering their costs and shifting the supply curve downward (rightward).
补贴是政府向生产者支付的款项，降低其成本，使供给曲线下移（右移）。
The subsidy per unit = pre‑ e − price received by producers after subsidy (or the vertical shift of the supply curve).
单位补贴 = 补贴前均衡价格 - 生产者实际获得的价格（或供给曲线的垂直下移幅度）。
The benefit is split between consumers and producers according to elasticities, with the more inelastic side gaining a larger share.
补贴利益按弹性在消费者和生产者之间分配，弹性更小的一方获益更多。
8. The Multiplier Effect | 乘数效应
The multiplier shows how an initial injection of spending leads to a larger final increase in national income.
乘数反映初始支出注入如何导致国民收入发生更大的最终增加。
Multiplier (k) = 1 / (1 − MPC) = 1 / MPW
save, MPT is marginal tax rate, MPM is marginal propensity to import. Also MPC + MPW = 1.
MPC 为边际消费倾向；MPW（边际漏出倾向）= MPS + MPT + MPM，其中 MPS 为边际储蓄倾向，MPT 边际税率，MPM 边际进口倾向。同时 MPC + MPW = 1。
The final change in national income = k × initial injection.
国民收入的最终变动 = 乘数 × 初始注入额。
9. Aggregate Demand and Aggregate Supply | 总需求与
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