📚 Year 12 Edexcel Economics: Formula & Theorem Quick Reference Handbook | Edexcel Year 12 经济：公式定理速查手册
This handbook compiles all the essential formulas, equations, and key economic theorems required for the Year 12 Edexcel Economics specification. It serves as a quick reference for Theme 1 (Introduction to Markets and Market Failure) and Theme 2 (The UK Economy – Performance and Policies). Each concept is presented with its English definition followed by the Chinese translation, making it easy for bilingual learners to revise efficiently.
本手册汇集了 Edexcel Year 12 经济学教学大纲所需的所有基本公式、等式和关键经济定理。它是主题 1（市场与市场失灵导论）和主题 2（英国经济——表现与政策）的快速参考。每个概念都先用英文定义再用中文翻译，方便双语学习者高效复习。
1. Price Elasticity of Demand (PED) | 需求价格弹性
Price elasticity of demand measures the responsiveness of quantity demanded to a change in price. The formula is: PED = (% change in quantity demanded) ÷ (% change in price). Using symbols, this is often written as %ΔQd / %ΔP.
需求价格弹性衡量需求量对价格变动的反应程度。公式为：PED = (需求量变动百分比) ÷ (价格变动百分比)。通常用符号表示为 %ΔQd / %ΔP。
A PED value greater than 1 (in absolute terms) indicates elastic demand, meaning a change in price leads to a more than proportionate change in quantity demanded. When PED is less than 1, demand is inelastic. A PED equal to exactly 1 is unit elastic, and a PED of infinity represents perfectly elastic demand, while zero shows perfectly inelastic demand.
如果 PED 的绝对值大于 1，需求富有弹性，意味着价格变化导致需求量更大幅度的变化。当 PED 小于 1 时，需求缺乏弹性。PED 恰好等于 1 代表单位弹性；PED 无穷大为完全弹性；PED 为零则完全无弹性。
For a downward-sloping linear demand curve, PED varies along the curve. It is elastic at high prices and inelastic at low prices. The relationship between PED and total revenue is crucial: if demand is elastic, a price fall raises total revenue; if inelastic, a price fall reduces total revenue.
对于向下倾斜的线性需求曲线，PED 沿着曲线变化：高价处富有弹性，低价处缺乏弹性。PED 与总收益的关系至关重要：若需求富有弹性，降价会增加总收益；若缺乏弹性，降价会减少总收益。
2. Income Elasticity of Demand (YED) | 需求收入弹性
Income elasticity of demand shows how quantity demanded responds to a change in consumer income. The formula is: YED = (% change in quantity demanded) ÷ (% change in income), or %ΔQd / %ΔY.
需求收入弹性表示需求量对消费者收入变化的反应程度。公式为：YED = (需求量变动百分比) ÷ (收入变动百分比)，或 %ΔQd / %ΔY。
Goods with a positive YED are normal goods. Among these, necessities have a YED between 0 and 1, while luxury goods have a YED greater than 1. Inferior goods have a negative YED, meaning demand falls as income rises.
YED 为正的商品是正常品。其中，必需品的 YED 在 0 到 1 之间，奢侈品 YED 大于 1。低档品的 YED 为负，即需求随收入增加而下降。
Firms use YED to forecast changes in demand during different stages of the economic cycle. For example, luxury car producers may expect strong sales growth when the economy is booming, while producers of inferior goods may benefit during a recession.
企业利用 YED 预测经济周期不同阶段的需求变化。例如，豪华车制造商在经济繁荣时可望销量强劲增长，而低档品生产商可能在经济衰退中受益。
3. Cross Elasticity of Demand (XED) | 需求交叉弹性
Cross elasticity of demand measures the responsiveness of demand for one good (A) to a change in the price of another good (B). The formula is: XED = (% change in quantity demanded of A) ÷ (% change in price of B), or %ΔQd(A) / %ΔP(B).
需求交叉弹性衡量一种商品 (A) 的需求量对另一种商品 (B) 价格变化的反应程度。公式为：XED = (A 需求量变动百分比) ÷ (B 价格变动百分比)，或 %ΔQd(A) / %ΔP(B)。
If XED is positive, the two goods are substitutes (e.g., Pepsi and Coca-Cola). If XED is negative, the two goods are complements (e.g., printers and ink cartridges). If XED is approximately zero, the goods are unrelated.
若 XED 为正，两种商品互为替代品（如百事可乐与可口可乐）。若 XED 为负，两种商品互为互补品（如打印机与墨盒）。若 XED 接近于零，则两种商品不相关。
XED is particularly useful for firms that operate in highly competitive markets. A firm can use XED to assess the likely impact of a rival’s price cut on its own sales, helping to shape pricing strategy.
XED 对身处高度竞争市场的企业尤为有用。企业可利用 XED 评估竞争对手降价对其自身销量的可能影响，从而制定定价策略。
4. Price Elasticity of Supply (PES) | 供给价格弹性
Price elasticity of supply measures the responsiveness of quantity supplied to a change in price. The formula is: PES = (% change in quantity supplied) ÷ (% change in price), or %ΔQs / %ΔP.
供给价格弹性衡量供给量对价格变动的反应程度。公式为：PES = (供给量变动百分比) ÷ (价格变动百分比)，或 %ΔQs / %ΔP。
Supply is price elastic when PES > 1, meaning producers can increase output quickly in response to a price rise. Supply is price inelastic when PES < 1. Key determinants of PES include the production time period, spare capacity, the ease of storing stock, and the flexibility of the production process.
当 PES > 1 时供给富有弹性，即生产者能迅速增加产出以回应价格上涨。当 PES < 1 时供给缺乏弹性。PES 的主要决定因素包括生产的时间跨度、闲置产能、库存保管的难易程度以及生产流程的灵活性。
In the momentary period, supply is typically perfectly inelastic (PES = 0). In the short run, at least one factor of production is fixed, so supply is relatively inelastic. In the long run, all factors are variable and supply becomes more elastic.
在瞬时期间，供给通常完全无弹性 (PES = 0)。短期内至少一种生产要素固定，因此供给相对缺乏弹性。长期内所有要素均可变，供给变得更加富有弹性。
5. Consumer Surplus, Producer Surplus, and Welfare Calculation | 消费者剩余、生产者剩余与总福利
Consumer surplus (CS) is the difference between what consumers are willing to pay and what they actually pay. In a linear demand and supply model, it can be expressed as the area of a triangle: CS = ½ × (maximum willingness to pay – equilibrium price) × equilibrium quantity.
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。在简单的线性供需模型中，它可以表示为三角形面积：CS = ½ × (最高支付意愿 – 均衡价格) × 均衡数量。
Producer surplus (PS) is the difference between the market price and the minimum price at which producers are willing to sell. Its area is: PS = ½ × (equilibrium price – minimum willingness to sell) × equilibrium quantity.
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。其面积为：PS = ½ × (均衡价格 – 最低销售意愿) × 均衡数量。
Total welfare is the sum of consumer and producer surplus: Total welfare = CS + PS. In a perfectly competitive market without externalities, total welfare is maximised at the free-market equilibrium. Any distortion, such as a tax or a monopoly, creates a deadweight loss that reduces total welfare.
总福利是消费者剩余与生产者剩余之和：总福利 = CS + PS。在一个没有外部性的完全竞争市场中，总福利在自由市场均衡点达到最大。任何一种扭曲，如税收或垄断，都会造成无谓损失，减少总福利。
6. Taxes, Subsidies, and Market Equilibrium | 税收、补贴与价格决定
A specific tax (a fixed amount per unit) shifts the supply curve vertically upwards by the amount of the tax. The new equilibrium price is higher and the quantity is lower. The tax burden is shared between consumers and producers. Consumer incidence = (Pb – Pe) × Q2, and producer incidence = (Pe – Ps) × Q2, where Pb is the price buyers pay, Pe is the original equilibrium price, Ps is the price sellers receive, and Q2 is the new quantity.
从量税（每单位征收固定金额）使供给曲线垂直上移税款金额。新的均衡价格更高，数量更低。税负由消费者和生产者共同分担。消费者负担 = (Pb – Pe) × Q2，生产者负担 = (Pe – Ps) × Q2，其中 Pb 是买方支付价格，Pe 是原均衡价格，Ps 是卖方实收价格，Q2 为新数量。
The deadweight loss (DWL) from a specific tax represents the loss of total welfare that is not transferred to the government. It is calculated as: DWL = ½ × tax per unit × (Q1 – Q2), where Q1 is the original equilibrium quantity.
从量税造成的无谓损失，代表未转化为政府税收的那部分总福利损失。计算公式为：DWL = ½ × 单位税款 × (Q1 – Q2)，其中 Q1 为原均衡数量。
A subsidy (a fixed payment per unit) shifts the supply curve downwards by the amount of the subsidy. The new equilibrium price is lower and quantity is higher. The deadweight loss from a subsidy, due to over-production, is given by: DWL = ½ × subsidy per unit × (Q2 – Q1).
补贴（每单位固定支付）使供给曲线下移补贴金额。新的均衡价格更低，数量更高。补贴因过度生产而产生的无谓损失为：DWL = ½ × 单位补贴额 × (Q2 – Q1)。
7. The Multiplier Effect | 乘数效应
The multiplier effect explains how an initial injection into the circular flow of income leads to a greater final increase in national income. The basic formula is: k = 1 / (1 – MPC), where MPC is the marginal propensity to consume, the fraction of any additional income that is spent on domestic goods and services.
乘数效应解释初始注入资金如何通过收入循环流导致国民收入更大幅度的最终增加。基本公式为：k = 1 / (1 – MPC)，其中 MPC 是边际消费倾向，即额外收入中用于购买国内商品和服务的比例。
The multiplier can also be expressed using the marginal propensity to withdraw, MPW, where MPW = MPS + MPT + MPM. MPS is the marginal propensity to save, MPT is the marginal tax rate, and MPM is the marginal propensity to import. Then, k = 1 / MPW.
乘数也可用边际漏出倾向 (MPW) 表示，其中 MPW = MPS + MPT + MPM。MPS 是边际储蓄倾向，MPT 是边际税率，MPM 是边际进口倾向。因此，k = 1 / MPW。
The total change in national income (ΔY) is calculated as: ΔY = k × ΔJ, where ΔJ is the initial change in an injection such as investment, government spending, or exports.
国民收入的全部变化 (ΔY) 计算公式为：ΔY = k × ΔJ，其中 ΔJ 是初始注入变量的变动，如投资、政府支出或出口。
8. Aggregate Demand and Macroeconomic Equilibrium | 总需求与宏观经济均衡
Aggregate demand (AD) is the total spending
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