📚 IB & Edexcel Economics: Formula Handbook | IB与Edexcel经济：公式汇总手册
Mastering key formulae is essential for success in both IB and Edexcel Economics. This handbook consolidates the most important calculations you will encounter across microeconomics and macroeconomics. Use it as a quick reference to strengthen your quantitative skills and boost your exam confidence.
掌握核心公式是学好 IB 和 Edexcel 经济学的关键。这份手册汇总了你在微观与宏观经济学中会遇到的最重要的计算。把它当作速查手册，帮你强化量化能力，提升考试信心。
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. It is calculated using the percentage changes in quantity demanded and price.
需求价格弹性衡量需求量对价格变动的反应程度。它通过需求量和价格变化的百分比来计算。
PED = %ΔQd ÷ %ΔP
If PED > 1, demand is price elastic; if PED < 1, demand is price inelastic; and if PED = 1, demand is unit elastic. When using the midpoint (arc) method, the formula changes to [(Q2 − Q1) / midQ] ÷ [(P2 − P1) / midP], where midQ = (Q1 + Q2) / 2 and midP = (P1 + P2) / 2.
如果 PED > 1，需求富有弹性；如果 PED < 1，需求缺乏弹性；如果 PED = 1，需求具有单位弹性。使用中点（弧）法时，公式变为 [(Q2 − Q1) / 中Q] ÷ [(P2 − P1) / 中P]，其中中Q = (Q1 + Q2) / 2，中P = (P1 + P2) / 2。
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 consumers’ income. It helps classify goods as normal, inferior, or luxury.
需求收入弹性表明需求量如何随消费者收入变化而变化。它有助于将商品分为正常品、低档品或奢侈品。
YED = %ΔQd ÷ %ΔY
If YED > 0, the good is a normal good; if YED < 0, it is an inferior good. A YED > 1 indicates a luxury good, while a YED between 0 and 1 suggests a necessity.
如果 YED > 0，该商品为正常品；如果 YED < 0，则为低档品。YED > 1 表示奢侈品，YED 在 0 到 1 之间则属于必需品。
3. Cross Elasticity of Demand (XED) | 需求交叉弹性
Cross elasticity of demand measures the sensitivity of demand for one good to a change in the price of another good. It identifies substitute and complementary relationships.
需求交叉弹性衡量一种商品的需求对另一种商品价格变化的敏感程度。它能识别替代品和互补品的关系。
XED = %ΔQd of good A ÷ %ΔP of good B
If XED > 0, the goods are substitutes; if XED < 0, they are complements. The larger the absolute value, the stronger the relationship.
如果 XED > 0，两种商品互为替代品；如果 XED < 0，则互为互补品。绝对值越大，关系越强。
4. Price Elasticity of Supply (PES) | 供给价格弹性
Price elasticity of supply reflects how quantity supplied changes in response to a price change. It depends on production flexibility, time period, and spare capacity.
供给价格弹性反映供给量对价格变化的反应程度。它取决于生产灵活性、时间周期和剩余产能。
PES = %ΔQs ÷ %ΔP
A PES > 1 indicates elastic supply, while PES < 1 indicates inelastic supply. In the long run, supply tends to be more elastic because firms can adjust all inputs.
PES > 1 表示供给富有弹性，PES < 1 表示供给缺乏弹性。长期中，由于厂商可以调整所有投入品，供给往往更有弹性。
5. Costs, Revenue and Profit | 成本、收益与利润
Firms use a set of formulae to track their financial performance. Understanding cost and revenue structures is crucial for profit maximisation and market analysis.
企业使用一系列公式追踪财务表现。理解成本与收益结构对于利润最大化和市场分析至关重要。
Total Cost (TC) is the sum of total fixed cost (TFC) and total variable cost (TVC). Average total cost (ATC) is TC divided by output (Q). Marginal cost (MC) is the extra cost from producing one more unit.
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) 等于 TC 除以产量 (Q)。边际成本 (MC) 是多生产一单位产品所带来的额外成本。
TC = TFC + TVC ATC = TC ÷ Q MC = ΔTC ÷ ΔQ
Total revenue (TR) equals price (P) multiplied by quantity sold (Q). Profit is TR − TC, and profit per unit is AR − ATC. A firm should continue producing as long as MR ≥ MC in the short run.
总收益 (TR) 等于价格 (P) 乘以销售量 (Q)。利润等于 TR − TC，单位利润为 AR − ATC。在短期内，只要边际收益 MR ≥ MC，企业就应继续生产。
6. Break-even Analysis | 盈亏平衡分析
Break-even analysis determines the level of output at which total revenue equals total cost, meaning the firm makes zero economic profit. It is a vital tool for business planning.
盈亏平衡分析确定总收益等于总成本时的产量水平，此时企业没有经济利润。它是企业规划的重要工具。
Break-even quantity = TFC ÷ (Price − AVC)
Contribution per unit is the difference between selling price and average variable cost. The margin of safety is the difference between actual output and break-even output, showing how much sales can fall before losses occur.
单位贡献毛利是售价与平均可变成本之差。安全边际是实际产出与盈亏平衡产出之间的差额，表明在出现亏损之前销售额可以下降多少。
7. Productivity and Efficiency | 生产率与效率
Productivity measures output per unit of input. Labour productivity and capital productivity are the most common indicators at the firm and economy levels.
生产率衡量每单位投入的产出。劳动生产率和资本生产率是企业和经济层面最常见的指标。
Labour productivity = Total output ÷ Number of workers
Efficiency concepts include productive efficiency (producing at lowest average cost) and allocative efficiency (price equals marginal cost). In macroeconomic contexts, productivity growth is a driver of long-run economic growth and rising living standards.
效率概念包括生产效率（以最低平均成本生产）和配置效率（价格等于边际成本）。在宏观经济背景下，生产率增长是长期经济增长和生活水平提高的驱动力。
8. Macroeconomic Indicators | 宏观经济指标
Macroeconomic analysis relies on a set of key indicators. GDP, the inflation rate, and the unemployment rate are the most frequently tested in IB and Edexcel exams.
宏观经济分析依赖一系列关键指标。GDP、通货膨胀率和失业率是 IB 和 Edexcel 考试中最常考查的指标。
GDP (expenditure method) = C + I + G + (X − M)
The GDP deflator is a measure of the price level: GDP deflator = (Nominal GDP ÷ Real GDP) × 100. Inflation rate is the percentage change in a price index, usually the Consumer Price Index (CPI). The unemployment rate is (Unemployed ÷ Labour force) × 100.
GDP 平减指数衡量价格水平：GDP平减指数 = (名义 GDP ÷ 实际 GDP) × 100。通货膨胀率是一个价格指数（通常是消费者价格指数 CPI）的变化百分比。失业率是（失业人数 ÷ 劳动力）× 100。
9. Multiplier and Aggregate Demand | 乘数与总需求
The multiplier effect captures how an initial injection into the circular flow leads to a larger final increase in national income. Understanding the multiplier is critical for evaluating fiscal policy.
乘数效应描述最初注入经济循环流的资金如何导致国民收入出现更大的最终增长。理解乘数对于评估财政政策至关重要。
k = 1 ÷ (1 − MPC) or k = 1 ÷ MPS
Where MPC is the marginal propensity to consume and MPS is the marginal propensity to save (MPC + MPS = 1). If the government increases spending by £10 billion and MPC = 0.8, the total increase in GDP = £10bn × 5 = £50bn. The multiplier can also be adjusted for taxes and imports: k = 1 ÷ (MPS + MPT + MPM).
其中 MPC 为边际消费倾向，MPS 为边际储蓄倾向（MPC + MPS = 1）。如果政府增加支出 100 亿英镑且 MPC = 0.8，GDP 总增加额为 100 亿 × 5 = 500 亿英镑。乘数还可以根据税收和进口进行调整：k = 1 ÷ (MPS + MPT + MPM)。
10. International Trade and Finance | 国际贸易与金融
Formulae in international economics help explain patterns of trade and exchange rate movements. Comparative advantage, terms of trade, and exchange rate conversions are common quantitative tasks.
国际经济学中的公式有助于解释贸易格局和汇率波动。比较优势、贸易条件和汇率换算是常见的定量任务。
Terms of trade index = (Export price index ÷ Import price index) × 100
A rise in the terms of trade means a country can buy more imports with a given quantity of exports. For exchange rates, the real exchange rate is RER = (Nominal exchange rate × Domestic price) ÷ Foreign price. When calculating comparative advantage, compare opportunity costs: the country with the lower opportunity cost in producing a good should specialise in it.
贸易条件指数上升意味着一国可以用既定数量的出口购买更多进口商品。在汇率方面，实际汇率公式为 RER =（名义汇率 × 国内价格）÷ 国外价格。计算比较优势时，应比较机会成本：生产某种商品机会成本较低的国家应专门生产该商品。
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