📚 A-Level AQA Economics: Formula Summary Handbook | A-Level AQA 经济：公式汇总手册
Mastering key formulas is essential for success in A-Level AQA Economics. Whether tackling microeconomic calculations like elasticities and costs, or macroeconomic concepts such as multipliers and output gaps, a solid grasp of these equations will empower you to analyse data, evaluate policies, and score highly on quantitative questions. This handbook brings together all the essential formulas you need to memorise and apply, organised for quick reference and revision.
掌握核心公式是 A-Level AQA 经济学取得成功的关键。无论是需求弹性与成本等微观计算，还是乘数与产出缺口等宏观概念，扎实掌握这些方程式都将帮助你分析数据、评估政策，并在量化题目中取得高分。本手册汇集了所有必须记忆和运用的重要公式，按模块整理，便于快速查阅和复习。
1. Price Elasticity of Demand (PED) | 需求的价格弹性
PED measures the responsiveness of quantity demanded to a change in price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price. Always interpret the absolute value; a value greater than 1 indicates elastic demand, less than 1 inelastic demand, and exactly 1 unitary elastic demand.
需求的价格弹性（PED）衡量需求量对价格变动的反应程度。它的计算公式是：需求量变动的百分比除以价格变动的百分比。通常使用绝对值进行解释；数值大于 1 为富有弹性，小于 1 为缺乏弹性，等于 1 为单位弹性。
PED = % change in quantity demanded ÷ % change in price
PED = 需求量变动百分比 ÷ 价格变动百分比
The midpoint (arc) elasticity formula is sometimes used to avoid inconsistency when moving up or down a demand curve: (ΔQ ÷ average Q) ÷ (ΔP ÷ average P). This is particularly useful for larger changes. In AQA examinations, you may be asked to interpret elasticity coefficients from data tables or graphs.
有时为了避免沿着需求曲线上移或下移时的不一致，会使用中点（弧）弹性公式：(ΔQ ÷ 平均 Q) ÷ (ΔP ÷ 平均 P)。该公式特别适用于较大幅度的变化。在 AQA 考试中，你可能需要根据数据表或图表解释弹性系数。
2. Income Elasticity of Demand (YED) | 需求的收入弹性
YED shows how quantity demanded responds to a change in consumers' income. A positive YED indicates a normal good; a negative YED indicates an inferior good. For normal goods, a YED greater than 1 is a luxury good, while a YED between 0 and 1 is a necessity.
YED 反映需求量如何随消费者收入变化而变化。YED 为正数表示正常品；为负数则表示低档品。就正常品而言，YED 大于 1 为奢侈品，介于 0 和 1 之间则为必需品。
YED = % change in quantity demanded ÷ % change in income
YED = 需求量变动百分比 ÷ 收入变动百分比
3. Cross Elasticity of Demand (XED) | 需求的交叉弹性
XED measures the responsiveness of demand for one good to a change in the price of another good. A positive XED identifies substitute goods, while a negative XED identifies complementary goods. The magnitude indicates the strength of the relationship. A value close to zero suggests the goods are unrelated.
XED 衡量一种商品的需求量对另一种商品价格变动的反应程度。XED 为正数时两者为替代品，为负数时则为互补品。数值大小反映了关系的强弱，接近零则表明两种商品不相关。
XED = % change in quantity demanded of Good A ÷ % change in price of Good B
XED = 商品 A 需求量变动百分比 ÷ 商品 B 价格变动百分比
4. Price Elasticity of Supply (PES) | 供给的价格弹性
PES indicates how responsive quantity supplied is to a change in price. In the short run, supply tends to be more inelastic, while in the long run it becomes more elastic as firms can adjust all factors of production. A PES less than 1 is inelastic, greater than 1 is elastic, and equal to 1 is unitary elastic.
PES 表示供给量对价格变动的反应程度。短期中供给通常更缺乏弹性，长期中企业能够调整所有生产要素，供给弹性因此增大。PES 小于 1 为缺乏弹性，大于 1 为富有弹性，等于 1 为单位弹性。
PES = % change in quantity supplied ÷ % change in price
PES = 供给量变动百分比 ÷ 价格变动百分比
5. Costs: Total, Average, Marginal | 成本：总成本、平均成本、边际成本
Firms face various cost structures. Total cost (TC) consists of total fixed cost (TFC) plus total variable cost (TVC). Average costs are derived by dividing by output (Q). Marginal cost (MC) is the extra cost of producing one more unit.
企业面临多种成本结构。总成本（TC）由总固定成本（TFC）和总可变成本（TVC）构成。平均成本可通过除以产量（Q）得到。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
总成本 = 总固定成本 + 总可变成本
AFC = TFC ÷ Q | AVC = TVC ÷ Q | ATC = TC ÷ Q or ATC = AFC + AVC
平均固定成本（AFC）= TFC ÷ Q | 平均可变成本（AVC）= TVC ÷ Q | 平均总成本（ATC）= TC ÷ Q 或 ATC = AFC + AVC
MC = ΔTC ÷ ΔQ
边际成本 = 总成本变动量 ÷ 产量变动量
The relationship between marginal and average costs is critical: when MC is below ATC, ATC falls; when MC is above ATC, ATC rises. This explains the U-shape of the average cost curve.
边际成本与平均成本之间的关系至关重要：当 MC 低于 ATC 时，ATC 下降；当 MC 高于 ATC 时，ATC 上升。这解释了平均成本曲线呈 U 形的原因。
6. Revenue: Total, Average, Marginal | 收益：总收益、平均收益、边际收益
Revenue concepts are at the heart of pricing and output decisions. Total revenue (TR) equals price times quantity sold. Average revenue (AR) equals TR divided by quantity, which is the demand curve. Marginal revenue (MR) is the change in TR from selling one more unit.
收益概念是定价与产量决策的核心。总收益（TR）等于价格乘以销售数量。平均收益（AR）等于总收益除以数量，也就是需求曲线。边际收益（MR）是多销售一单位产品带来的总收益变化。
TR = P × Q
总收益 = 价格 × 数量
AR = TR ÷ Q = P
平均收益 = 总收益 ÷ 数量 = 价格
MR = ΔTR ÷ ΔQ
边际收益 = 总收益变动量 ÷ 数量变动量
e AR (demand) curve in linear demand models. Understanding this helps predict profit-maximising output where MR = MC.
在完全竞争中，由于企业是价格接受者，AR = MR。在不完全竞争下，MR 位于 AR 下方，且在线性需求模型中 MR 曲线的斜率是 AR（需求）曲线的两倍。理解这一点有助于预测利润最大化产量（MR = MC 之处）。
7. Profit Calculations | 利润计算
Economists distinguish between accounting profit and economic profit. Accounting profit is TR minus explicit costs, while economic profit subtracts both explicit and implicit (opportunity) costs. Normal profit occurs when TR equals total economic cost, meaning zero economic profit. Supernormal profit exists when TR exceeds total cost.
经济学家区分会计利润和经济利润。会计利润是总收益减去显性成本，而经济利润则要减去显性成本和隐性（机会）成本。当总收益等于总经济成本时，企业获得正常利润，即经济利润为零。当总收益超过总成本时，存在超常利润。
Accounting Profit = TR − Explicit Costs
会计利润 = 总收益 − 显性成本
Economic Profit = TR − (Explicit Costs + Implicit Costs)
经济利润 = 总收益 − (显性成本 + 隐性成本)
rom below. In diagrams, this determines the optimal output level. AQA questions frequently require candidates to shade profit areas or identify profit-maximising price and quantity.
任何企业的利润最大化产量均位于 MC = MR 且 MC 从下方穿过 MR 之处。在图形中，这决定了最优产出水平。AQA 题目经常要求考生给利润区域涂色或找出利润最大化时的价格与产量。
8. Market Concentration Ratios | 市场集中度比率
Concentration ratios measure the combined market share of the largest firms in an industry. The most common is the N-firm concentration ratio (e.g. CR5 or CR4). A high ratio signals an oligopolistic or monopolistic market. AQA may also refer to the Herfindahl-Hirschman Index (HHI) as an alternative measure, which squares and sums the market shares of all firms.
集中度比率衡量行业中最大几家企业的市场份额总和。最常见的为 N 企业集中度比率（如 CR5 或 CR4）。比率较高表明市场为寡头垄断或完全垄断。AQA 也可能提及赫芬达尔—赫希曼指数（HHI）作为替代指标，该指数将各企业市场份额进行平方后求和。
CRn = (S₁ + S₂ + … + Sₙ) ÷ Total Market Sales × 100
CRn = (企业 1 销售额 + … + 企业 n 销售额) ÷ 市场总销售额 × 100
HHI = Σ (Market Share of each firm)²
HHI = Σ (每家企业的市场份额)²
A CR5 above 60% typically suggests a concentrated oligopoly. HHI values above 2,000 indicate high concentration, while values below 1,000 suggest a relatively unconcentrated market. These tools are essential for evaluating competition and market power.
CR5 高于 60% 通常表明高集中度的寡头市场。HHI 值高于 2000 表示高度集中，低于 1000 则表示市场相对分散。这些工具对于评估竞争程度和市场支配力至关重要。
9. Macroeconomic Multipliers | 宏观经济乘数
The multiplier effect explains how an initial injection into the circular flow leads to a larger final increase in national income. Its size depends on the marginal propensity to consume (MPC), save (MPS), tax (MPT), and import (MPM). AQA often requires calculation of the simple multiplier and interpretation of its implications for fiscal policy.
乘数效应解释了一笔初始注入如何通过循环流转带来国民收入的更大增长。乘数大小取决于边际消费倾向（MPC）、边际储蓄倾向（MPS）、边际税收倾向（MPT）和边际进口倾向（MPM）。AQA 经常要求计算简单乘数，并解读其对财政政策的意义。
Simple Multiplier = 1 ÷ (1 − MPC) = 1 ÷ MPS
简单乘数 = 1 ÷ (1 − MPC) = 1 ÷ MPS
Full Multiplier = 1 ÷ (MPS + MPT + MPM)
完整乘数 = 1 ÷ (MPS + MPT + MPM)
A higher MPC (or lower leakages) yields a larger multiplier. This amplifies the impact of government spending or investment changes on GDP. In evaluation, candidates should discuss factors like spare capacity, time lags, and the state of the economy that affect the multiplier's real-world effectiveness.
较高的 MPC（或较少的漏出）会产生更大的乘数。这会放大政府支出或投资变动对 GDP 的影响。在评估时，考生应讨论闲置产能、时间滞后和经济状况等因素如何影响乘数在现实中的有效性。
10. GDP and Real GDP | 国内生产总值与实际国内生产总值
Gross Domestic Product measures the total value of output produced within an economy. Nominal GDP uses current prices, while real GDP adjusts for inflation, giving a more accurate picture of economic growth. The GDP deflator and Consumer Price Index (CPI) are common tools for converting nominal to real values.
国内生产总值衡量一国经济中产出的总价值。名义 GDP 使用当期价格，而实际 GDP 剔除了通货膨胀的影响，能更准确地反映经济增长。GDP 平减指数和消费者物价指数（CPI）是将名义值转换为实际值的常用工具。
GDP = C + I + G + (X − M)
GDP = 消费 + 投资 + 政府支出 + (出口 − 进口)
Real GDP = (Nominal GDP ÷ Price Index) × 100
实际 GDP = (名义 GDP ÷ 价格指数) × 100
growth is typically presented as the annual percentage change in real GDP. AQA questions may ask you to index a data series or compute growth rates from tables.
GDP 平减指数的公式为 (名义 GDP ÷ 实际 GDP) × 100。经济增长通常表现为实际 GDP 的年度百分比变化。AQA 考题可能会要求你对数据序列进行指数化处理，或根据表格计算经济增长率。
11. Exchange Rates and Terms of Trade | 汇率与贸易条件
Exchange rates express the price of one currency in terms of another. Their calculation may involve converting from one base to another, often using given cross rates. The trade-weighted exchange rate index reflects a weighted average of a currency's value against a basket of trading partners' currencies.
汇率表示一国货币用另一国货币衡量的价格。计算时可涉及将一种货币换算为另一种，常使用给定的交叉汇率。贸易加权汇率指数则反映一国货币相对于一篮子贸易伙伴货币的加权平均值。
00. Appreciation means a rise in value, depreciation a fall. The terms of trade index compares export and import prices, providing insight into trade gains and living standards.
汇率变动百分比的计算公式为 (新值 − 旧值) ÷ 旧值 × 100。升值指货币价值上升，贬值指下降。贸易条件指数对比出口价格与进口价格，可反映贸易利得和生活水平。
Terms of Trade Index = (Index of Export Prices ÷ Index of Import Prices) × 100
贸易条件指数 = (出口价格指数 ÷ 进口价格指数) × 100
An improvement in the terms of trade (a rise in the index) means a country can buy more imports for a given volume of exports. However, this may reduce export competitiveness. Students should be able to interpret movements and link them to exchange rate changes.
贸易条件改善（指数上升）意味着一国给定出口量能换取更多的进口品。然而，这可能削弱出口竞争力。学生需能够解读指数变动，并将其与汇率变化联系起来。
12. Real Interest Rates and Output Gap | 实际利率与产出缺口
The real interest rate is a key determinant of saving, investment, and consumption. It strips out inflation to show the true return on savings or cost of borrowing. Central banks monitor this closely when setting policy rates.
实际利率是储蓄、投资与消费的重要决定因素。它剔除了通货膨胀的影响，以显示储蓄的真实回报或借款的真实成本。央行在设定政策利率时会密切关注该指标。
Real Interest Rate = Nominal Interest Rate − Inflation Rate
实际利率 = 名义利率 − 通货膨胀率
The output gap measures the difference between actual output and potential output, expressed as a percentage of potential GDP. A positive output gap indicates an overheating economy with inflationary pressures; a negative gap signals spare capacity and unemployment.
产出缺口衡量实际产出与潜在产出之间的差额，以潜在 GDP 的百分比表示。正产出缺口表明经济过热，存在通胀压力；负产出缺口则表明存在闲置产能和失业。
Output Gap = (Actual GDP − Potential GDP) ÷ Potential GDP × 100
产出缺口 = (实际 GDP − 潜在 GDP) ÷ 潜在 GDP × 100
This concept is connected to the Phillips curve, unemployment, and fiscal policy debates. In AQA macro exams, you may be asked to illustrate the output gap on an AD-AS diagram or discuss its implications for demand-side and supply-side policies.
这一概念与菲利普斯曲线、失业和财政政策辩论相关联。在 AQA 宏观考试中，你可能需要运用 AD-AS 图形显示产出缺口，或讨论其对需求侧和供给侧政策的影响。
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