📚 Year 13 Edexcel Economics: Formula & Theorem Quick Reference | Year 13 Edexcel 经济：公式定理速查手册
Having a solid grasp of key formulas and theorems is essential for success in Year 13 Edexcel Economics. This quick reference guide consolidates the most important equations, conditions and theoretical relationships across microeconomics and macroeconomics, helping you to apply them accurately under exam conditions.
扎实掌握关键公式定理对于 Year 13 Edexcel 经济学的成功至关重要。这份速查手册汇集了微观与宏观经济学中最重要的方程式、条件与理论关系，帮助你在考试环境下准确应用。
1. Short‑Run Cost Relationships | 短期成本关系
Total cost (TC) is the sum of total fixed cost (TFC) and total variable cost (TVC). Average total cost (ATC) is found by dividing TC by output (Q).
总成本（TC）是总固定成本（TFC）与总可变成本（TVC）之和。平均总成本（ATC）由 TC 除以产量（Q）得出。
TC = TFC + TVC
ATC = TC ÷ Q
Marginal cost (MC) is the change in TC when output changes by one unit. MC cuts both ATC and AVC at their minimum points.
边际成本（MC）是增加一单位产量所带来的总成本变化。MC 在平均总成本和平均可变成本的最低点分别与其相交。
MC = ΔTC ÷ ΔQ
2. Revenue and Profit Maximisation | 收益与利润最大化
Total revenue (TR) equals price (P) multiplied by quantity sold (Q). Average revenue (AR) equals price and the demand curve.
总收益（TR）等于价格（P）乘以销售量（Q）。平均收益（AR）等于价格，也就是需求曲线。
TR = P × Q
AR = TR ÷ Q = P
Profit is maximised where marginal revenue (MR) equals marginal cost (MC). A firm will continue to expand output as long as MR > MC.
当边际收益（MR）等于边际成本（MC）时利润最大化。只要 MR > MC，企业就会继续扩大产出。
Profit maximisation: MR = MC
3. Perfect Competition and Allocative Efficiency | 完全竞争与配置效率
In perfect competition, the equilibrium occurs where price equals marginal cost, delivering allocative efficiency. Productive efficiency requires output at the lowest point of the ATC curve.
在完全竞争中，均衡发生在价格等于边际成本之时，从而实现配置效率。生产效率则要求产出位于平均总成本曲线的最低点。
Allocative efficiency: P = MC
Productive efficiency: P = minimum ATC
In the long run, perfectly competitive firms earn only normal profit because any supernormal profit attracts new entrants, shifting industry supply rightwards and lowering price until P = ATC = MC.
长期内，完全竞争企业只能获得正常利润，因为任何超额利润都会吸引新进入者，使行业供给右移，压低价格，直至 P = ATC = MC。
4. Monopoly and Price Discrimination | 垄断与价格歧视
A pure monopolist faces a downward‑sloping demand curve, so marginal revenue is less than price. Profit‑maximising output is still where MR = MC, but the price is set on the demand curve above that output.
纯粹垄断者面临向下倾斜的需求曲线，因此边际收益小于价格。利润最大化产量仍位于 MR = MC，但价格由该产量对应的需求曲线上的点决定。
Third‑degree price discrimination charges different prices to groups with different price elasticities of demand. A necessary condition is that the markets can be separated and resale is impossible.
三级价格歧视对具有不同需求价格弹性的群体收取不同价格。必要条件之一是市场能够被分割，且无法转售。
P > MC in monopoly, creating deadweight loss
5. Oligopoly: Concentration and Kinked Demand | 寡头：集中度与弯折需求
The n‑firm concentration ratio adds up the market shares of the largest n firms. A high ratio signals an oligopolistic market structure.
n 企业集中度比率是将最大的 n 家企业的市场份额相加。高集中度比率表明市场结构为寡头垄断。
CRₙ = Σ (sales of top n firms) ÷ total market sales
The kinked demand curve model assumes firms will match price cuts but not price rises, leading to a discontinuous MR curve and price rigidity.
弯折需求曲线模型假设企业会跟随降价但不会跟随涨价，导致边际收益曲线出现断裂，价格因而呈刚性。
6. Labour Market: MRP and Wage Determination | 劳动力市场：MRP 与工资决定
A firm’s demand for labour is derived from the marginal revenue product of labour (MRPL), which equals marginal product of labour (MPL) multiplied by marginal revenue (MR).
企业对劳动的需求源于劳动的边际收益产品（MRPL），它等于劳动的边际产品（MPL）乘以边际收益（MR）。
MRPL = MPL × MR
In a perfectly competitive labour market, a firm hires workers up to the point where MRPL equals the wage rate (W). Monopsony employers, however, hire less labour and pay a wage below MRPL.
在完全竞争的劳动力市场上，企业雇用工人的数量会使 MRPL 等于工资率（W）。然而买方垄断雇主会雇佣较少劳动，并支付低于 MRPL 的工资。
Profit‑maximising employment: MRPL = W (for competitive labour markets)
7. The Multiplier and Aggregate Demand | 乘数与总需求
The multiplier (k) measures the final change in real GDP resulting from an initial injection into the circular flow. Its size depends on the marginal propensities to consume (MPC), save (MPS), tax (MPT) and import (MPM).
乘数（k）衡量的是初始注入循环流后实际 GDP 的最终变化。其大小取决于边际消费倾向（MPC）、边际储蓄倾向（MPS）、边际税收倾向（MPT）和边际进口倾向（MPM）。
k = 1 ÷ (1 − MPC) = 1 ÷ MPW = 1 ÷ (MPS + MPT + MPM)
A higher MPC (or lower withdrawals) leads to a larger multiplier. The final change in GDP equals the initial injection times the multiplier.
MPC 越高（或漏出越低），乘数越大。GDP 的最终变化等于初始注入乘以乘数。
ΔY = k × ΔJ
The aggregate demand curve shows the relationship between the price level and real GDP, and it shifts with changes in consumption, investment, government spending or net exports.
总需求曲线表示物价水平与实际 GDP 之间的关系，它会因消费、投资、政府支出或净出口的变化而移动。
8. The Phillips Curve and the Output Gap | 菲利普斯曲线与产出缺口
The short‑run Phillips curve shows an inverse relationship between inflation and unemployment. In the long run, however, the Phillips curve is vertical at the natural rate of unemployment (NAIRU).
短期菲利普斯曲线显示通胀与失业之间的反向关系。然而，长期菲利普斯曲线在自然失业率（NAIRU）处呈垂直状态。
Short‑run trade‑off: ↓ unemployment ↔ ↑ inflation
A positive output gap occurs when actual GDP exceeds potential GDP, creating inflationary pressure; a negative output gap occurs when actual GDP is below potential, leading to cyclical unemployment.
当实际 GDP 超过潜在 GDP 时，出现正产出缺口，形成通胀压力；当实际 GDP 低于潜在 GDP 时，出现负产出缺口，导致周期性失业。
9. Comparative Advantage and Trade | 比较优势与贸易
Comparative advantage exists when a country can produce a good at a lower opportunity cost than another country. The opportunity cost of good X is the amount of good Y sacrificed divided by the amount of X gained.
当一个国家生产一种商品的机会成本低于另一国家时，就存在比较优势。商品 X 的机会成本等于牺牲的商品 Y 数量除以获得的 X 数量。
Opportunity cost of X = ΔY ÷ ΔX
Complete specialisation and trade according to comparative advantage can allow both countries to consume outside their production possibility frontiers.
按照比较优势实行完全专业化分工和贸易，可以使两国的消费越过其生产可能性边界。
10. Exchange Rates and the Balance of Payments | 汇率与国际收支
An appreciation of the currency makes exports more expensive and imports cheaper, worsening the current account in the short run (the J‑curve effect) but potentially improving it in the long run if the Marshall–Lerner condition holds.
本币升值使出口更贵、进口更便宜，短期内会恶化经常账户（J 曲线效应），但如果满足马歇尔‑勒纳条件，长期则可能改善经常账户。
Marshall–Lerner condition: |PEDₓ| + |PEDₘ| > 1
The balance of payments must always balance because the sum of the current account, capital account and financial account, plus the net errors and omissions, is zero.
国际收支必定平衡，因为经常账户、资本账户与金融账户之和加上净误差与遗漏始终为零。
11. Economic Growth and the Harrod‑Domar Model | 经济增长与哈罗德‑多马模型
Actual economic growth can be measured by the percentage change in real GDP. Potential growth depends on increases in the quantity and quality of factors of production.
实际经济增长可通过实际 GDP 的百分比变动来衡量。潜在增长则取决于生产要素数量和质量的提升。
Growth rate = (GDPₜ − GDPₜ₋₁) ÷ GDPₜ₋₁ × 100
The Harrod‑Domar model links growth to the savings ratio (s) and the capital‑output ratio (κ). Developing countries often need to raise savings to accelerate growth.
哈罗德‑多马模型将增长与储蓄率（s）和资本‑产出比（κ）联系起来。发展中国家通常需要提高储蓄才能加速增长。
Growth rate (g) = s ÷ κ
12. Measures of Inequality and Development | 不平等与发展指标
The Gini coefficient is derived from the Lorenz curve and measures income inequality. A value of 0 indicates perfect equality, while 1 indicates perfect inequality.
基尼系数由洛伦兹曲线得出，用于衡量收入不平等。0 表示完全平等，1 表示完全不平等。
Gini coefficient = A ÷ (A + B)
The Human Development Index (HDI) combines life expectancy, education (mean and expected years of schooling) and GNI per capita (PPP). It provides a broader measure of development than GDP alone.
人类发展指数（HDI）综合了预期寿命、教育（平均受教育年限和预期受教育年限）以及按购买力平价衡量的人均国民总收入。它比单纯的 GDP 更能全面衡量发展水平。
HDI = (Iₕ + Iₑ + Iᵧ) ÷ 3
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