📚 Year 12 WJEC Economics: Formulas & Theorems Quick Reference | WJEC AS经济公式定理速查手册
This handbook compiles the essential formulas and key economic theorems required for Year 12 WJEC Economics. Whether revising for unit tests or preparing for AS assessments, these concise references cover microeconomic and macroeconomic principles, elasticity calculations, multiplier effects, and policy trade-offs. Each entry is presented with clear English definitions followed by Chinese explanations, making it a practical bilingual revision tool.
这本手册汇编了WJEC Year 12经济学所需的基本公式和关键经济定理。无论是单元测试复习还是AS评估备考，这些简明参考涵盖微观与宏观经济原理、弹性计算、乘数效应及政策权衡。每个条目均提供清晰的英文定义与中文解释，成为实用的双语复习工具。
1. Demand, Supply and Market Equilibrium | 需求、供给与市场均衡
The law of demand: Ceteris paribus, as the price of a good falls, the quantity demanded increases. The demand function is often written as Qd = a − bP, where a is the intercept and b is the slope.
需求定律：在其他条件不变时，商品价格下降，需求量上升。需求函数通常写作 Qd = a − bP，其中 a 为截距，b 为斜率。
Determinants of demand include income, tastes, prices of substitutes and complements, and population. A shift in demand changes the ‘a’ parameter.
需求的决定因素包括收入、偏好、替代品和互补品价格以及人口。需求的变化会改变参数 a。
The law of supply: Ceteris paribus, as the price of a good rises, the quantity supplied increases. Supply function: Qs = c + dP, where d is positive.
供给定律：在其他条件不变时，商品价格上升，供给量增加。供给函数：Qs = c + dP，其中 d 为正。
Market equilibrium occurs where Qd = Qs, determining the equilibrium price P* and quantity Q*. Disequilibrium creates shortages (excess demand) or surpluses (excess supply).
市场均衡在 Qd = Qs 时实现，决定均衡价格 P* 和均衡数量 Q*。非均衡状态产生短缺（超额需求）或过剩（超额供给）。
2. Elasticities: PED, PES, YED, XED | 弹性：需求价格弹性、供给价格弹性、收入弹性、交叉弹性
Price elasticity of demand (PED) measures responsiveness of quantity demanded to a price change. Formula: PED = %ΔQd / %ΔP. Using the midpoint method: PED = (ΔQd / average Qd) ÷ (ΔP / average P).
需求价格弹性 (PED) 衡量需求量对价格变化的反应程度。公式：PED = %ΔQd / %ΔP。中点法公式：PED = (ΔQd / 平均 Qd) ÷ (ΔP / 平均 P)。
- |PED| > 1: elastic demand (需求量变化弹性充足) — total revenue moves inversely with price.
- |PED| > 1: 富有弹性 — 价格与总收益反向变动。
- |PED| < 1: inelastic demand (缺乏弹性) — total revenue moves with price.
- |PED| < 1: 缺乏弹性 — 价格与总收益同向变动。
- |PED| = 1: unit elastic (单位弹性) — total revenue stays constant when price changes.
- |PED| = 1: 单位弹性 — 价格变化时总收益不变。
Price elasticity of supply (PES) = %ΔQs / %ΔP. Determinants include production time lag, spare capacity, and perishability.
供给价格弹性 (PES) = %ΔQs / %ΔP。决定因素包括生产时滞、闲置产能和易腐性。
Income elasticity of demand (YED) = %ΔQd / %ΔY (income). Normal goods have YED > 0; inferior goods have YED < 0. Luxury goods YED > 1.
需求的收入弹性 (YED) = %ΔQd / %ΔY (收入)。正常品 YED > 0；低档品 YED < 0；奢侈品 YED > 1。
Cross elasticity of demand (XED) = %ΔQd of good A / %ΔP of good B. Substitutes have XED > 0; complements have XED < 0.
需求的交叉弹性 (XED) = A 商品需求量变化百分比 / B 商品价格变化百分比。替代品 XED > 0；互补品 XED < 0。
3. 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 the market price. It is the area below the demand curve and above the market price. For a linear demand curve, consumer surplus = ½ × base × height.
消费者剩余是消费者愿意支付的最高价格与市场价格之间的差额，表示为需求曲线以下、市场价格以上的面积。线性需求曲线下，消费者剩余 = ½ × 底 × 高。
Producer surplus is the difference between the market price and the minimum price producers are willing to accept. It is the area above the supply curve and below the market price. Producer surplus = ½ × base × height for a linear supply curve.
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，表示为供给曲线以上、市场价格以下的面积。线性供给曲线下，生产者剩余 = ½ × 底 × 高。
Total surplus = consumer surplus + producer surplus. At competitive equilibrium, total surplus is maximised, indicating allocative efficiency (P = MC).
总剩余 = 消费者剩余 + 生产者剩余。在竞争性均衡点，总剩余最大化，表明配置效率（P = MC）。
4. Market Failure and Government Intervention | 市场失灵与政府干预
Market failure occurs when the free market fails to allocate resources efficiently. Key types: externalities, public goods, information gaps, and monopoly power.
市场失灵指自由市场未能有效配置资源。主要类型包括外部性、公共品、信息不对称和垄断力量。
For negative production externalities, social cost = private cost + external cost. A Pigouvian tax equal to the marginal external cost can internalise the externality.
对于负生产外部性，社会成本 = 私人成本 + 外部成本。与边际外部成本相等的庇古税可以内部化外部性。
For positive consumption externalities, social benefit = private benefit + external benefit. A subsidy equal to the marginal external benefit can correct under-consumption.
对于正消费外部性，社会收益 = 私人收益 + 外部收益。等于边际外部收益的补贴可以纠正消费不足。
Public goods are non-rival and non-excludable, leading to the free-rider problem. Government often provides them directly.
公共品具有非竞争性和非排他性，导致搭便车问题。政府通常直接提供公共品。
The Coase theorem suggests that if property rights are well defined and transaction costs are low, private bargaining can solve externalities without government intervention.
科斯定理表明，若产权界定清晰且交易成本低，私人谈判可在无需政府干预下解决外部性问题。
5. Macroeconomic Objectives and Indicators | 宏观经济目标与指标
Key macroeconomic objectives: low and stable inflation, high employment, sustainable economic growth, and a satisfactory balance of payments.
主要宏观经济目标：低而稳定的通胀、高就业、可持续经济增长以及令人满意的国际收支。
Inflation rate (CPI based) = [(CPIcurrent − CPIprevious) / CPIprevious] × 100%. The CPI measures changes in the cost of a typical basket of goods.
通货膨胀率（基于 CPI）= [(CPI当前 − CPI前期) / CPI前期] × 100%。CPI 衡量一篮子典型商品成本的变化。
Unemployment rate = (number of unemployed / labour force) × 100%. The labour force includes those employed plus those actively seeking work.
失业率 = (失业人数 / 劳动力人数) × 100%。劳动力包括就业者和积极寻找工作的人。
Economic growth rate = [(GDPcurrent − GDPprevious) / GDPprevious] × 100%. Real GDP strips out the effect of inflation.
经济增长率 = [(GDP当前 − GDP前期) / GDP前期] × 100%。实际 GDP 剔除了通胀的影响。
6. Aggregate Demand and the Multiplier | 总需求与乘数效应
Aggregate demand (AD) = C + I + G + (X − M). C is consumption, I investment, G government spending, X exports, M imports.
总需求 (AD) = C + I + G + (X − M)。C 为消费，I 投资，G 政府支出，X 出口，M 进口。
The marginal propensity to consume (MPC) = ΔC / ΔY, where Y is disposable income. Multiplier k = 1 / (1 − MPC) or k = 1 / (MPS + MPT + MPM), where MPS is marginal propensity to save, MPT marginal propensity to tax, MPM marginal propensity to import.
边际消费倾向 (MPC) = ΔC / ΔY，Y 为可支配收入。乘数 k = 1 / (1 − MPC) 或 k = 1 / (MPS + MPT + MPM)，MPS 为边际储蓄倾向，MPT 边际税率，MPM 边际进口倾向。
The multiplier effect: an initial injection of spending ΔJ leads to a larger final increase in GDP: ΔGDP = k × ΔJ. The size of the multiplier depends on leakages.
乘数效应：一笔初始注入支出 ΔJ 会导致 GDP 更大规模的最终增加：ΔGDP = k × ΔJ。乘数大小取决于漏出量。
7. Aggregate Supply and Macroeconomic Equilibrium | 总供给与宏观经济均衡
Short-run aggregate supply (SRAS) is upward sloping because input prices (especially wages) are sticky. In the short run, firms can increase output in response to higher prices.
短期总供给曲线 (SRAS) 向上倾斜，因为投入价格（尤其是工资）具有粘性。短期内企业可因价格上涨而提高产出。
Long-run aggregate supply (LRAS) is vertical at the full-employment level of output, indicating that in the long run output depends on factor endowments and technology, not the price level.
长期总供给曲线 (LRAS) 在充分就业产出水平处垂直，表明长期产出取决于要素禀赋和技术，而非价格水平。
Macroeconomic equilibrium occurs where AD = SRAS. A positive demand shock shifts AD right, raising price and output in the short run. Supply-side shocks shift SRAS.
宏观经济均衡在 AD = SRAS 时实现。正需求冲击使 AD 右移，短期内提高价格和产出。供给侧冲击会使 SRAS 移动。
The output gap is the difference between actual GDP and potential GDP (LRAS). A negative output gap indicates spare capacity and unemployment.
产出缺口是实际 GDP 与潜在 GDP (LRAS) 之差。负产出缺口表明存在闲置产能和失业。
8. International Trade and Exchange Rates | 国际贸易与汇率
Comparative advantage: A country has a comparative advantage in producing a good if it can produce it at a lower opportunity cost than another country. Specialisation and trade can increase total output.
比较优势：如果一国生产某种商品的机会成本低于另一国，则该国在生产该商品上具有比较优势。专业化与贸易可以提高总产出。
Terms of trade index = (index of export prices / index of import prices) × 100. An improvement in the terms of trade means a country can buy more imports for a given volume of exports.
贸易条件指数 = (出口价格指数 / 进口价格指数) × 100。贸易条件改善意味着一定量的出口可以换回更多的进口。
Exchange rate: the price of one currency in terms of another, e.g. £1 = $1.25. Appreciation means the currency becomes stronger; depreciation means it becomes weaker.
汇率：一种货币以另一种货币表示的价格，例如 £1 = $1.25。升值意味着货币走强；贬值意味着货币走弱。
Real exchange rate = (nominal exchange rate × domestic price level) / foreign price level. It measures competitiveness.
实际汇率 = (名义汇率 × 国内价格水平) / 国外价格水平，用于衡量竞争力。
Current account deficits and surpluses affect exchange rates. A deficit may cause depreciation, while a surplus may cause appreciation, under floating exchange rate systems.
经常账户赤字和盈余影响汇率。在浮动汇率制下，赤字可能导致贬值，盈余可能导致升值。
9. Costs, Revenues and Profit Maximisation | 成本、收益与利润最大化
Total cost (TC) = total fixed cost (TFC) + total variable cost (TVC). Average cost (AC) = TC / Q. Marginal cost (MC) = ΔTC / ΔQ.
总成本 (TC) = 总固定成本 (TFC) + 总可变成本 (TVC)。平均成本 (AC) = TC / Q。边际成本 (MC) = ΔTC / ΔQ。
Total revenue (TR) = Price × Quantity. Average revenue (AR) = TR / Q = Price (for a price-taking firm). Marginal revenue (MR) = ΔTR / ΔQ.
总收益 (TR) = 价格 × 数量。平均收益 (AR) = TR / Q = 价格（对于价格接受者）。边际收益 (MR) = ΔTR / ΔQ。
Profit maximisation condition: produce where MR = MC. If MR > MC, a firm can increase profit by raising output. If MR < MC, it should reduce output.
利润最大化条件：在 MR = MC 处生产。若 MR > MC，增加产出可提高利润；若 MR < MC，应减少产出。
Normal profit occurs when AR = AC (or TR = TC). This is the minimum reward needed to keep resources in their current use. Supernormal profit is AR > AC.
正常利润出现在 AR = AC (或 TR = TC) 时，这是维持资源现有用途所需的最低回报。超额利润指 AR > AC。
Shutdown point in the short run: a firm should continue operating if P ≥ AVC, because it can cover variable costs and contribute to fixed costs.
短期停业点：若 P ≥ AVC，企业应继续经营，因为它可以弥补可变成本并对固定成本有所贡献。
10. The Phillips Curve and Policy Trade-offs | 菲利普斯曲线与政策权衡
The original Phillips curve shows an inverse relationship between wage inflation and unemployment. The modern short-run Phillips curve (SRPC) depicts a trade-off between price inflation and unemployment.
原始菲利普斯曲线显示工资通胀与失业之间反向关系。现代短期菲利普斯曲线 (SRPC) 描绘了物价通胀与失业之间的权衡。
In the long run, the Phillips curve is vertical at the natural rate of unemployment (NAIRU), meaning there is no permanent trade-off between inflation and unemployment.
长期菲利普斯曲线在自然失业率 (NAIRU) 处垂直，意味着通胀与失业之间不存在长期权衡。
An demand-side expansion may move the economy along the SRPC, reducing unemployment but raising inflation. Supply-side improvements can shift the LRAS and the Phillips curve to the left, lowering both inflation and unemployment.
需求扩张可能使经济沿短期菲利普斯曲线移动，降低失业但推高通胀。供给侧改善可使 LRAS 和菲利普斯曲线左移，同时降低通胀与失业。
Policy trade-offs: using monetary or fiscal policy to boost AD might reduce unemployment at the cost of higher inflation. Policymakers must balance these objectives.
政策权衡：利用货币或财政政策刺激总需求可能以减少失业为代价推高通胀。政策制定者必须平衡这些目标。
The expectations-augmented Phillips curve incorporates inflation expectations. If expected inflation rises, the SRPC shifts right, worsening the trade-off.
附加预期的菲利普斯曲线纳入了通胀预期。若预期通胀上升，短期菲利普斯曲线右移，恶化权衡关系。
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