📚 AS Edexcel Economics: Key Formulas and Theorems Quick Guide | AS Edexcel 经济：公式定理速查手册
This quick reference handbook collates the essential formulas and core theorems required for the AS Edexcel Economics specification. It covers both microeconomic relationships such as elasticity concepts, surplus measures, tax incidence and profit maximisation, as well as macroeconomic fundamentals including the multiplier and terms of trade. Each section pairs a concise English explanation with a Chinese translation to support bilingual learning and exam preparation.
本速查手册汇集了 AS Edexcel 经济学考试规格所需的基本公式与核心定理。内容涵盖微观经济关系，如各类弹性概念、剩余度量、税收分担与利润最大化，以及宏观经济基础，包括乘数和贸易条件。每个部分均以简洁的英文解释搭配中文译文，助力双语学习与备考。
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, calculated as the percentage change in quantity demanded divided by the percentage change in price. The formula is:
需求价格弹性衡量需求量对价格变化的反应程度，计算为需求量变动百分比除以价格变动百分比。公式为：
PED = (% ΔQd) ÷ (% ΔP)
If PED > 1, demand is price elastic; if PED < 1, demand is price inelastic; if PED = 1, demand is unit elastic. Knowledge of the determinants (substitutes, necessity vs luxury, time period, proportion of income) and its implication for total revenue is crucial.
若 PED > 1，需求富有价格弹性；若 PED < 1，需求缺乏价格弹性；若 PED = 1，需求为单位弹性。了解其决定因素（替代品、必需品与奢侈品、时间跨度、收入占比）及其对总收益的影响至关重要。
2. Income Elasticity of Demand (YED) | 需求收入弹性
Income elasticity of demand measures how quantity demanded changes in response to a change in income. It is computed by:
需求收入弹性衡量需求量如何随收入变化而变动，计算公式为：
YED = (% ΔQd) ÷ (% ΔY)
Normal goods have positive YED (YED > 0); inferior goods have negative YED. Among normal goods, those with YED > 1 are income elastic (luxuries), while those with 0 < YED < 1 are income inelastic (necessities). This helps firms forecast demand over the business cycle.
正常品具有正的收入弹性（YED > 0）；劣质品则具有负的收入弹性。在正常品中，YED > 1 者为收入弹性商品（奢侈品），0 < YED < 1 者为收入缺乏弹性商品（必需品）。这有助于企业根据经济周期预测需求。
3. Cross Elasticity of Demand (XED) | 需求交叉弹性
Cross elasticity of demand indicates how the demand for one good reacts to a change in the price of another good. The formula is:
需求交叉弹性表明一种商品的需求如何因另一种商品价格变化而变动。公式为：
XED = (% ΔQd of good A) ÷ (% ΔP of good B)
If XED > 0, the two goods are substitutes; if XED < 0, they are complements. A value close to zero suggests weak or no relationship. Firms use XED to analyse competitive threats and pricing strategies for complementary goods.
若 XED > 0，则两种商品为替代品；若 XED < 0，则为互补品。数值接近零表示关系较弱或无关。企业使用交叉弹性分析竞争威胁及互补品的定价策略。
4. Price Elasticity of Supply (PES) | 供给价格弹性
Price elasticity of supply measures the responsiveness of quantity supplied to a change in price. It is defined as:
供给价格弹性衡量供给量对价格变动的反应程度，定义为：
PES = (% ΔQs) ÷ (% ΔP)
When PES > 1, supply is price elastic; when PES < 1, supply is price inelastic. Key determinants include production time period, spare capacity, ease of factor substitution, and inventory levels. In the short run, supply is often more inelastic.
当 PES > 1 时，供给富有价格弹性；当 PES < 1 时，供给缺乏价格弹性。关键决定因素包括生产时间周期、闲置产能、要素替代的难易程度及库存水平。短期中，供给往往更缺乏弹性。
5. Total Revenue and Elasticity | 总收益与弹性
Total revenue (TR) is the money a firm receives from sales, calculated as price multiplied by quantity sold:
总收益（TR）是企业从销售中获得的货币，等于价格乘以销售量：
TR = P × Q
The relationship between TR and PED is a vital theorem. When demand is elastic, a price cut raises total revenue; a price increase lowers it. When demand is inelastic, a price cut reduces total revenue; a price increase raises it. Revenue is maximised at the point of unit elasticity.
总收益与需求价格弹性的关系是一个重要定理。当需求富有弹性时，降价会提高总收益，涨价会降低总收益。当需求缺乏弹性时，降价减少总收益，涨价增加总收益。在单位弹性点收益最大化。
6. Consumer and Producer Surplus | 消费者剩余与生产者剩余
Formulas for consumer surplus (CS) and producer surplus (PS) are not fixed equations but geometric areas under demand and above supply curves. The underlying principle:
消费者剩余（CS）和生产者剩余（PS）的公式并非固定方程式，而是需求曲线下方、供给曲线上方的几何面积。基本原理：
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Consumer surplus = the area between the demand curve and the market price, representing the extra benefit consumers receive.
消费者剩余 = 需求曲线与市场价格之间的面积，代表消费者获得的额外利益。
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Producer surplus = the area between the market price and the supply curve, reflecting the extra revenue over marginal cost.
生产者剩余 = 市场价格与供给曲线之间的面积，反映超过边际成本的额外收入。
These surpluses are used to measure welfare changes due to shifts in supply and demand, indirect taxes, subsidies, and price controls.
这些剩余用于衡量由于供需变化、间接税、补贴及价格管制所导致的福利变化。
7. Tax and Subsidy Incidence | 税收与补贴分担
The burden of an indirect tax or the benefit of a subsidy is shared between consumers and producers. The incidence formula relies on relative elasticities:
间接税的负担或补贴的收益由消费者与生产者共同分担。分担公式基于相对弹性：
Consumer tax incidence = (PES ÷ (PES + PED)) × tax per unit
Producer tax incidence = (PED ÷ (PES + PED)) × tax per unit
The more inelastic side of the market bears a larger proportion. A subsidy works analogously: the gain enjoyed by each side depends similarly on PED and PES.
市场中弹性越小的一方承担越大比例。补贴的作用类似：各方享受的益处同样取决于需求价格弹性和供给价格弹性。
8. Marginal Principles in Production | 生产的边际原理
Marginal cost (MC) and average cost (AC) are central to production theory. While exact equations depend on total cost functions, the key relationships are:
边际成本（MC）与平均成本（AC）是生产理论的核心。虽然精确方程式取决于总成本函数，但关键关系为：
MC = ΔTC ÷ ΔQ
AC = TC ÷ Q
The theorem states: when MC < AC, AC is falling; when MC > AC, AC is rising; MC intersects AC at its minimum point. Similarly, marginal revenue (MR) is ΔTR ÷ ΔQ. Diminishing marginal returns cause MC eventually to rise.
定理指出：当 MC < AC 时，AC 下降；当 MC > AC 时，AC 上升；MC 在 AC 的最低点与之相交。同理，边际收益（MR）是 ΔTR ÷ ΔQ。边际报酬递减规律导致边际成本最终上升。
9. Profit Maximisation Rule | 利润最大化法则
A fundamental theorem of the firm is that profits are maximised (or losses minimised) at the output where marginal cost equals marginal revenue, provided MC cuts MR from below:
企业的一个基本定理是：在边际成本等于边际收益且边际成本从下方穿过边际收益的产量水平上，利润最大化（或损失最小化）：
MC = MR
For a perfectly competitive firm, MR equals the market price, so the rule becomes P = MC. Shut-down point occurs where price falls below average variable cost (P < AVC). In imperfect markets, the firm's ability to influence price makes MR ≠ P, yet the MC = MR rule still applies.
对于完全竞争企业，边际收益等于市场价格，因此法则变为 P = MC。当价格低于平均可变成本（P < AVC）时，企业停止营业。在不完全市场，企业影响价格的能力使 MR ≠ P，但 MC = MR 法则依然适用。
10. Multiplier Effect | 乘数效应
The multiplier quantifies the magnified impact of an initial injection into the circular flow on national income. The formulas are:
乘数量化了一笔初始注入到循环流转中对国民收入的放大影响。公式为：
Multiplier = 1 ÷ (1 − MPC) = 1 ÷ MPS
ΔNational Income = Multiplier × ΔInjections
where MPC is the marginal propensity to consume and MPS is the marginal propensity to save (or, more precisely, the marginal propensity to withdraw, combining savings, tax and imports). A higher MPC yields a larger multiplier.
其中 MPC 为边际消费倾向，MPS 为边际储蓄倾向（或更准确地说是边际撤出倾向，综合储蓄、税收和进口）。MPC 越高，乘数越大。
11. Terms of Trade | 贸易条件
The terms of trade index measures a country’s export prices relative to import prices. An improvement implies the country can buy more imports for a given volume of exports. The formula is:
贸易条件指数衡量一国出口价格相对于进口价格的水平。改善意味着以既定出口量可购买更多进口品。公式为：
Terms of Trade Index = (Index of Export Prices ÷ Index of Import Prices) × 100
Factors affecting the terms of trade include changes in global demand/supply for key exports, exchange rate movements, and relative inflation rates. A rise in the index indicates an improvement, while a fall indicates a deterioration.
影响贸易条件的因素包括全球对主要出口品供需的变化、汇率波动以及相对通胀率。指数上升表明改善，下降表明恶化。
12. Unemployment and Output Gaps | 失业与产出缺口
While not a single precise formula at AS, the relationship between cyclical unemployment and the output gap is often expressed through a version of Okun’s law. A negative output gap (actual GDP below potential GDP) is associated with higher cyclical unemployment:
虽然在 AS 阶段没有单一精确公式，但周期性失业与产出缺口之间的关系常通过奥肯定律的某种形式表达。负产出缺口（实际 GDP 低于潜在 GDP）与更高的周期性失业相关：
Output Gap (%) = (Actual GDP − Potential GDP) ÷ Potential GDP × 100
Knowledge of this indicator helps analyse the performance of an economy and the need for demand‑side or supply‑side policies. A positive output gap may create inflationary pressure.
了解这一指标有助于分析经济运行状况以及是否需要需求端或供给端政策。正产出缺口可能带来通胀压力。
Published by TutorHao | Economics Revision Series | aleveler.com
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