📚 IGCSE WJEC Economics: Formula Summary Handbook | IGCSE WJEC 经济：公式汇总手册
Mastering key formulas is the backbone of success in IGCSE WJEC Economics. This handbook brings together every essential calculation you need – from the arc of elasticity to national income aggregates – with paired explanations in English and Chinese to reinforce understanding.
掌握关键公式是在 IGCSE WJEC 经济学中取得成功的基石。本手册汇集了从弹性弧到国民收入总量等所有必要计算方法，并配有中英文对照讲解，以巩固理解。
1. Percentage Change Basics | 百分比变化基础
Percentage change is used throughout economic analysis to measure how much a variable has increased or decreased relative to its original value. The standard formula divides the change by the start value and multiplies by 100.
百分比变化广泛应用于经济分析中，用以衡量变量相对于其原始值的增减幅度。标准公式为变化量除以初始值再乘以100。
% Change = (New Value − Old Value) ÷ Old Value × 100
A positive result signals growth; a negative result indicates a contraction. This metric underpins every elasticity and growth rate you will calculate.
正数结果表示增长；负数结果表示收缩。该指标是计算所有弹性和增长率的基础。
For example, if a price rises from £10 to £12, the percentage increase is (12 − 10) ÷ 10 × 100 = 20%. Always express percentage changes to one or two decimal places for accuracy.
例如，价格从10英镑涨到12英镑，百分比涨幅为 (12 − 10) ÷ 10 × 100 = 20%。为精确起见，百分比变化通常保留一或两位小数。
2. Price Elasticity of Demand (PED) | 需求价格弹性
PED measures the responsiveness of quantity demanded to a change in price. It always takes a negative value because of the law of demand, but economists often compare its absolute size.
PED 衡量需求量对价格变动的反应程度。由于需求定律，它总是取负值，但经济学家通常比较其绝对值大小。
PED = %ΔQd ÷ %ΔP
When |PED| > 1, demand is elastic: consumers are sensitive to price changes. When |PED| < 1, demand is inelastic. Unit elasticity occurs when |PED| = 1, where total revenue stays constant as price changes.
当 |PED| > 1 时，需求富有弹性：消费者对价格变化敏感。当 |PED| < 1 时，需求缺乏弹性。单位弹性出现在 |PED| = 1 时，此时价格变化后总收入保持不变。
Determinants include the availability of substitutes, the proportion of income spent, and whether the good is a necessity or luxury. A perfectly elastic demand curve is horizontal (PED = ∞); perfectly inelastic demand is vertical (PED = 0).
影响 PED 的因素包括替代品的可获得性、支出占收入的比例以及商品是必需品还是奢侈品。完全弹性需求曲线为水平线（PED = ∞）；完全无弹性需求曲线为垂直线（PED = 0）。
3. Price Elasticity of Supply (PES) | 供给价格弹性
PES shows how much quantity supplied changes when price alters. It is usually positive because higher prices encourage more output.
PES 表明价格变动时供给量的变化程度。它通常为正，因为较高的价格鼓励更多的产出。
PES = %ΔQs ÷ %ΔP
If PES > 1, supply is elastic; producers can raise output quickly. If PES < 1, supply is inelastic, often due to limited capacity, long production times, or scarce inputs. A PES of 0 represents perfectly inelastic supply (immediate run), while a horizontal supply curve signals perfect elasticity.
若 PES > 1，供给富有弹性；生产者可迅速提高产量。若 PES < 1，供给缺乏弹性，通常是由于产能有限、生产周期长或投入稀缺。PES 为 0 表示完全无弹性供给（瞬时），而水平供给曲线则代表完全弹性。
Key influences include time period, spare capacity, complexity of production, and storability of the product. Understanding PES helps explain why agricultural prices can be volatile.
主要影响因素包括时间段、闲置产能、生产复杂性和产品的可储存性。理解 PES 有助于解释农产品价格为何容易波动。
4. Income Elasticity of Demand (YED) | 需求收入弹性
YED captures how demand shifts in response to changes in consumer income. It helps classify goods as normal or inferior.
YED 反映了需求如何随消费者收入变化而变动，有助于将商品划分为正常品或劣等品。
YED = %ΔQd ÷ %ΔY
For normal goods, YED is positive. Necessities have a YED between 0 and 1, while luxury goods show YED > 1. Inferior goods have a negative YED, as demand falls when incomes rise.
正常品的 YED 为正。必需品的 YED 介于 0 到 1 之间，而奢侈品的 YED > 1。劣等品的 YED 为负，因为收入增加时需求反而下降。
Businesses use YED to forecast sales during economic cycles. A high income elasticity indicates a sector that will boom during expansions but suffer sharply in recessions.
企业利用 YED 来预测经济周期中的销售情况。高收入弹性表明该行业在扩张期会繁荣，但在衰退期会急剧萎缩。
5. Cross Elasticity of Demand (XED) | 需求交叉弹性
XED measures how the demand for one good responds to a price change in another good. It reveals the relationship between products.
XED 衡量一种商品的需求如何随另一种商品价格变动而变化，揭示了产品之间的关系。
XED = %ΔQdA ÷ %ΔPB
Substitute goods have a positive XED: a rise in the price of brand B increases demand for brand A. Complementary goods show a negative XED: when cameras become cheaper, demand for memory cards rises. Unrelated products have a near-zero XED.
替代品的 XED 为正：品牌 B 价格上涨会增加对品牌 A 的需求。互补品的 XED 为负：当相机变便宜时，对存储卡的需求上升。不相关的产品 XED 接近于零。
The magnitude of XED tells firms about competitive threats and partnership opportunities. A high positive XED means firms are close rivals; a strong negative XED suggests joint marketing potential.
XED 的大小告知企业竞争威胁与合作机会。较高的正 XED 意味着企业间是紧密对手；较强的负 XED 则暗示联合营销的潜力。
6. Price Index and Inflation | 价格指数与通胀
A price index, often the Consumer Price Index (CPI), tracks the overall cost of a fixed basket of goods and services over time. The base year is set to 100.
价格指数（通常指消费者价格指数 CPI）追踪一篮子固定商品和服务随时间推移的总成本。基年设定为 100。
CPI = (Cost of Basket in Current Year ÷ Cost of Basket in Base Year) × 100
Inflation rate is the percentage growth of the index from one period to the next. It measures the erosion of purchasing power.
通胀率是该指数从一个时期到下一个时期的百分比增长，衡量了购买力的侵蚀程度。
Inflation Rate = (CPIt − CPIt−1) ÷ CPIt−1 × 100
Governments and central banks monitor inflation closely. Moderate inflation is often a sign of a healthy economy, but hyperinflation or deflation can be damaging.
政府和中央银行密切关注通胀。温和通胀通常是经济健康的标志，但恶性通胀或通缩可能带来破坏性影响。
7. Unemployment and Labour Market | 失业与劳动力市场
The unemployment rate captures the share of the labour force that is actively seeking work but unable to find it. The labour force includes those employed plus the unemployed.
失业率反映了劳动人口中积极寻找工作但未找到的人群比例。劳动人口包括就业者和失业者。
Unemployment Rate = (Number Unemployed ÷ Labour Force) × 100
Not all working-age individuals are in the labour force; students, retirees, and discouraged workers may be excluded. The labour force participation rate measures engagement.
并非所有劳动适龄人口都处于劳动人口中；学生、退休人员和丧失信心的工人可能被排除在外。劳动参与率衡量了参与程度。
Participation Rate = (Labour Force ÷ Working-Age Population) × 100
Policymakers use these figures to gauge economic slack and design training or job-creation programmes. A falling participation rate can mask true unemployment.
政策制定者使用这些数据评估经济闲置情况，并设计培训或创造就业方案。参与率下降可能掩盖真实的失业状况。
8. National Income Accounting | 国民收入核算
Gross Domestic Product (GDP) sums the final market value of goods and services produced within a country in a given period. The expenditure method is the most common in IGCSE.
国内生产总值（GDP）加总了一国在特定时期内生产的最终商品和服务的市场价值。支出法在 IGCSE 中最常见。
GDP = C + I + G + (X − M)
C stands for household consumption, I for business investment, G for government spending, X for exports, and M for imports. Net exports (X − M) can be positive or negative.
C 代表家庭消费，I 代表企业投资，G 代表政府支出，X 代表出口，M 代表进口。净出口（X − M）可为正或负。
We can also measure national income using the income or output methods, but the aggregate should be identical. GDP excludes transfer payments and second-hand sales to avoid double counting.
我们也可以用收入法或产出法衡量国民收入，但总量应一致。GDP 排除转移支付和二手交易，以避免重复计算。
9. Economic Growth and Per Capita Measures | 经济增长与人均指标
Real GDP adjusts nominal GDP for inflation to reveal true output changes. It is calculated using a price deflator.
实际 GDP 将名义 GDP 根据通胀调整，以揭示真实的产出变化。计算时需使用价格平减指数。
Real GDP = (Nominal GDP ÷ GDP Deflator) × 100
Economic growth is the percentage rise in real GDP from one period to the next. It signals expanding productive capacity.
经济增长是指实际 GDP 从一个时期到下一个时期的百分比增长，标志着生产能力的扩大。
Growth Rate = (Real GDPt − Real GDPt−1) ÷ Real GDPt−1 × 100
GDP per capita divides national output by total population, showing average living standards. Though useful, it ignores income distribution, non-market activities, and environmental costs.
人均 GDP 是国民产出除以总人口数，用以显示平均生活水平。尽管实用，但它忽略了收入分配、非市场活动以及环境成本。
GDP per capita = GDP ÷ Population
10. Costs, Revenue and Profit | 成本、收入与利润
Firms need to understand cost structure to make pricing and output decisions. Total cost combines fixed costs (rent, salaries) with variable costs (materials, energy).
企业需要了解成本结构，以便做出定价和产量决策。总成本由固定成本（租金、薪金）和可变成本（原材料、能源）组成。
Total Cost (TC) = Fixed Cost (FC) + Variable Cost (VC)
Average cost is the expense per unit of output, while marginal cost is the extra cost of producing one more unit.
平均成本是每单位产出的费用，而边际成本是多生产一个单位所带来的额外成本。
Average Cost (AC) = TC ÷ Q
Marginal Cost (MC) = ΔTC ÷ ΔQ
Total revenue is the income from sales. Profit is the difference between total revenue and total cost; break-even occurs when TR = TC.
总收入是销售收入，利润是总收入与总成本之差；当 TR = TC 时达到盈亏平衡。
Total Revenue (TR) = Price × Quantity
Profit = TR − TC
11. Productivity and Efficiency | 生产率与效率
Productivity measures how efficiently inputs are converted into outputs. Labour productivity is the most common metric at IGCSE.
生产率衡量投入转化为产出的效率。劳动生产率为 IGCSE 中最常见的指标。
Labour Productivity = Total Output ÷ Number of Workers
It can also be expressed per worker hour. Rising productivity is the main source of long-run economic growth and higher real wages.
它也可以表示为每小时工作产出。生产率提升是长期经济增长和实际工资提高的主要来源。
Other measures include capital productivity (output per unit of capital) and multifactor productivity. Investments in technology, training, and infrastructure typically boost productivity.
其他指标包括资本生产率（每单位资本的产出）和全要素生产率。对技术、培训和基础设施的投资通常能提高生产率。
12. Exchange Rates and Trade | 汇率与贸易
Exchange rates convert one currency into another. If you know the direct quote, multiplying converts home currency to foreign currency; dividing does the reverse.
汇率可将一种货币兑换成另一种。若已知直接标价，乘以汇率可将本币转为外币，除以汇率则相反。
Foreign Currency Amount = Home Currency Amount × Exchange Rate
For example, if £1 = $1.25, £100 equals $125. A depreciation makes exports cheaper and imports dearer, while an appreciation has the opposite effect.
例如，若 £1 = $1.25，则 £100 等于 $125。货币贬值使出口更便宜、进口更贵，升值则相反。
The terms of trade index compares export prices to import prices, indicating how many imports can be bought with one unit of exports.
贸易条件指数比较了出口价格与进口价格，表明一单位出口能购买多少进口。
Terms of Trade = (Index of Export Prices ÷ Index of Import Prices) × 100
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