📚 Year 13 WJEC Economics: Formula & Theorem Quick Reference Handbook | Year 13 WJEC 经济：公式定理速查手册
This compact handbook brings together all the essential formulas, theorems and quantitative relationships you need to master for Year 13 WJEC Economics. Whether you are revising elasticities, crunching multiplier numbers or checking the conditions for profit maximisation, this guide will provide rapid recall in both English and Chinese. Use it alongside your class notes and past papers to build confidence for the A2 examinations.
这本速查手册汇集了 Year 13 WJEC 经济课程中所有你需掌握的关键公式、定理和数量关系。不论是复习弹性、计算乘数还是核查利润最大化条件，这本中英对照手册都能帮你快速回忆。请配合课堂笔记和历年真题使用，为 A2 考试树立信心。
1. Elasticity Formulas | 弹性公式
Price elasticity of demand (PED) measures the responsiveness of quantity demanded to a change in the good’s own price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price. The expression is usually written with the absolute value taken, but the co‑efficient is negative due to the law of demand.
PED = %ΔQd / %ΔP
需求价格弹性（PED）衡量需求量对商品自身价格变化的反应程度。它等于需求量变化百分比除以价格变化百分比。受需求定律影响，弹性系数通常为负，习惯上取其绝对值讨论。
PED = 需求量变动百分比 / 价格变动百分比
| |PED| > 1 | Price elastic | 弹性 >1 |
| |PED| = 1 | Unit elastic | 单位弹性 |
| |PED| < 1 | Price inelastic | 弹性 <1 |
| PED = 0 | Perfectly inelastic | 完全无弹性 |
| PED → ∞ | Perfectly elastic | 完全弹性 |
Income elasticity of demand (YED) links the change in quantity demanded to a change in consumers’ income. A positive YED signals a normal good, while a negative value indicates an inferior good. If YED exceeds 1, the good is a luxury.
YED = %ΔQd / %ΔY
需求收入弹性（YED）反映需求量对消费者收入变动的敏感度。YED为正时该商品为正常品，为负时则是低档品；若YED大于1，该商品属于奢侈品。
YED = 需求量变动百分比 / 收入变动百分比
Cross elasticity of demand (XED) captures how the quantity demanded of one good responds to a price change of another good. Substitutes carry positive XED values, while complements carry negative values.
XED = %ΔQd of good A / %ΔP of good B
交叉弹性（XED）衡量一种商品的需求量对另一种商品价格变动的反应。替代品的XED为正，互补品的XED为负。
XED = A商品需求量变动百分比 / B商品价格变动百分比
Price elasticity of supply (PES) gives the responsiveness of quantity supplied to a price change. It is always positive because of the law of supply. The magnitude indicates whether supply is elastic, inelastic or unit elastic.
PES = %ΔQs / %ΔP
供给价格弹性（PES）衡量供给量对价格变动的反应程度。由于供给定律，PES恒为正，其数值大小表明供给富有弹性、缺乏弹性或单位弹性。
PES = 供给量变动百分比 / 价格变动百分比
2. Cost, Revenue and Profit | 成本、收益与利润
Total cost (TC) splits into total fixed cost (TFC) and total variable cost (TVC). Average cost (AC) is total cost per unit, while average variable cost (AVC) and average fixed cost (AFC) are calculated analogously.
TC = TFC + TVC
AC = TC / Q
总成本由总固定成本和总可变成本构成。平均成本即单位成本，还可以进一步拆分为平均可变成本与平均固定成本。
总成本 = 总固定成本 + 总可变成本
平均成本 = 总成本 ÷ 产量
Marginal cost (MC) is the additional cost of producing one more unit. It plays a crucial role in supply decisions and is normally U‑shaped due to the law of diminishing returns.
MC = ΔTC / ΔQ
边际成本是增产一单位所带来的总成本增量。边际成本曲线通常呈U形，是利润最大化决策的关键依据。
MC = 总成本变动量 ÷ 产量变动量
Total revenue (TR) is price multiplied by quantity sold. Average revenue (AR) equals TR/Q and is identical to the demand curve. Marginal revenue (MR) is the change in TR from selling one extra unit.
TR = P × Q
AR = TR / Q = P
MR = ΔTR / ΔQ
总收益等于价格与销量的乘积。平均收益即单位收益，在图形上就是需求曲线。边际收益是增售一单位带来的总收益变化。
总收益 = 价格 × 销售量
平均收益 = 总收益 ÷ 销售量 = 价格
边际收益 = 总收益变动量 ÷ 销售量变动量
ld reduce output. Normal profit is earned when AR = AC, while supernormal profit arises when AR > AC.
Profit‑maximising condition: MR = MC
利润最大化条件为边际收益等于边际成本。若MR大于MC，继续增产能增加利润；若MR小于MC，则需减产。正常利润对应 AR = AC，超额利润则 AR > AC。
利润最大化条件：MR = MC
3. Market Structures: Revenue, Demand and Shut‑down | 市场结构：收益、需求线及停业点
In perfect competition the firm is a price‑ = MR = P. The demand curve is perfectly elastic. In a monopoly, the firm faces a downward‑ ies below AR. The relationship helps determine the price and output that maximise profits.
完全竞争市场中厂商是价格接受者，其 AR = MR = P，需求曲线是一条水平线。垄断厂商面临向下倾斜的需求曲线，因此边际收益位于平均收益下方。这些关系可以确定利润最大化的价格与产量。
he firm cannot cover its variable costs, it should cease production temporarily. The break‑ point occurs where AR (=P) equals average total cost (ATC), yielding zero economic profit.
短期停业点出现在价格低于平均可变成本时（P < AVC）。若企业连可变成本也无法弥补，则应暂时停产。盈亏平衡点位于平均收益（即价格）等于平均总成本处，此时经济利润为零。
In a contestable market, the threat of entry keeps prices close to average cost, even if the incumbent is a monopolist. The key is the absence of sunk costs, which allows hit‑and‑run entry.
在可竞争市场中，即使现存的是一家垄断企业，进入威胁也会迫使价格接近平均成本。关键在于不存在沉没成本，为“打完就跑”式的进入提供了可能。
4. Labour Market | 劳动力市场
Marginal physical product (MPP) is the extra output generated by an additional worker. Multiplying MPP by marginal revenue gives the marginal revenue product (MRP), which represents the value of the worker’s contribution.
MRP = MPP × MR
劳动的边际物质产品（MPP）是增加一名工人所带来额外产出。MPP乘以边际收益即为边际收益产品（MRP），代表该劳动者的价值贡献。
MRP = MPP × MR
market is perfectly competitive, MR equals price, so MRP simplifies to MPP × P. The profit‑ rginal cost of labour). In perfectly competitive labour markets, MCL equals the wage rate.
若产品市场是完全竞争的，MR等于价格，MRP可简化为MPP × P。利润最大化的雇工水平由 MRP = MCL（劳动的边际成本）决定。在完全竞争的劳动市场中，MCL就等于工资率。
Wage elasticity of demand for labour indicates how responsive firms’ labour demand is to a change in the wage rate. A more elastic labour demand makes it easier to substitute capital for labour.
劳动需求的工资弹性衡量企业用工量对工资率变动的反应。劳动需求弹性越大，用资本替代劳动就越容易。
5. National Income and the Multiplier | 国民收入与乘数
Aggregate demand (AD) is the total spending on domestic output at a given price level. Its components are consumption, investment, government spending and net exports.
AD = C + I + G + (X − M)
总需求（AD）是本国在特定价格水平下被购买的商品和服务的总支出，由消费、投资、政府支出和净出口构成。
总需求 = 消费 + 投资 + 政府支出 + (出口 − 进口)
The consumption function shows the relationship between disposable income (Yd) and planned consumption. Autonomous consumption (a) is spending when income is zero, while the marginal propensity to consume (MPC) is the fraction of extra income spent.
C = a + MPC × Yd
消费函数反映可支配收入与计划消费之间的关系。自发消费a是收入为零时的支出，边际消费倾向MPC则是新增收入中用于消费的比例。
C = a + MPC × 可支配收入
The Keynesian multiplier captures the magnified effect an initial injection has on national income. Its simplest form is the reciprocal of the marginal propensity to withdraw. When the economy is open and taxed, the multiplier shrinks.
k = 1 / (1 − MPC) = 1 / MPS
k = 1 / (MPS + MPT + MPM)
凯恩斯乘数反映初始注入对国民收入的放大效应，最简单的形式是边际漏出倾向的倒数。在开放且征税的经济中，乘数值变小。
k = 1 / (1 − MPC) = 1 / MPS
k = 1 / (MPS + MPT + MPM)
The accelerator principle links
Published by TutorHao | Year 13 Economics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图