A-Level WJEC Economics: Formula Summary Handbook | A-Level WJEC 经济：公式汇总手册

📚 A-Level WJEC Economics: Formula Summary Handbook | A-Level WJEC 经济：公式汇总手册

This handbook presents a comprehensive collection of essential formulas for the WJEC A-Level Economics specification. Understanding and applying these quantitative tools will strengthen your analysis, support your evaluation in essays, and boost your confidence in data-response questions.

本手册汇总了 WJEC A-Level 经济学科目所需的所有关键公式。熟练理解并运用这些量化工具，将加深你的分析能力，为论述题提供有力支撑，并显著提升你在数据分析题中的答题信心。

1. Elasticities | 弹性公式

Elasticity measures how responsive one variable is to a change in another. In WJEC Economics, you must be able to calculate, interpret and evaluate price, income, cross and supply elasticities.

弹性衡量一个变量对另一个变量变化的反应程度。WJEC 经济学要求你能够计算、解释并评价需求价格弹性、收入弹性、交叉弹性和供给弹性。

Formula	Description
PED = %ΔQ_d / %ΔP	Price elasticity of demand
YED = %ΔQ_d / %ΔY	Income elasticity of demand
XED = %ΔQ_dA / %ΔP_B	Cross elasticity of demand
PES = %ΔQ_s / %ΔP	Price elasticity of supply

PED measures the sensitivity of quantity demanded to a change in the good’s own price. A value above 1 (ignoring the negative sign) indicates elastic demand, while a value below 1 shows inelastic demand. Unitary elasticity occurs when PED = 1. WJEC often asks candidates to link PED to total revenue changes: when demand is elastic, a price cut raises total revenue.

PED 衡量需求量对自身价格变化的敏感度。绝对值大于1表示富有弹性，小于1表示缺乏弹性，等于1则为单位弹性。WJEC 常要求考生将 PED 与总收益变动联系起来：当需求富有弹性时，降价会提高总收益。

YED shows how demand reacts to a change in consumer income. A positive YED identifies a normal good; a negative YED reveals an inferior good. For normal goods, YED > 1 marks a luxury, while 0 < YED < 1 suggests a necessity.

YED 反映需求对消费者收入变动的反应。正收入弹性说明该商品是正常品，负收入弹性则是低档品。对于正常品，YED 大于1为奢侈品，介于0与1之间为必需品。

XED captures the relationship between two goods. A positive XED indicates substitutes, whereas a negative XED points to complements. The size of the coefficient shows the strength of the relationship, helping firms predict the impact of competitors’ pricing strategies.

XED 揭示两种商品之间的关系。正交叉弹性表明二者互为替代品，负值则为互补品。弹性数值的大小反映了关联强度，有助于企业预判竞争对手定价策略的影响。

PES measures producers’ responsiveness to price changes. Values greater than 1 show elastic supply, while values below 1 indicate inelastic supply. Determinants such as spare capacity, stock levels and time period directly affect PES and are frequently examined.

PES 衡量生产者对价格变动的反应程度。大于1表示供给富有弹性，小于1则缺乏弹性。闲置产能、库存水平和时间周期等决定因素直接影响 PES，也是考试常见要点。

2. Revenue, Costs and Profit | 收入、成本与利润

A firm’s objective, whether profit maximisation or another goal, can be analysed through revenue and cost formulas. WJEC requires you to calculate and diagram total, average and marginal concepts.

无论企业目标是利润最大化还是其他目标，都可以通过收入和成本公式进行分析。WJEC 要求你计算并绘制总收入、平均收入和边际收入以及各类成本的概念图。

Formula	Description
TR = P × Q	Total revenue
AR = TR / Q = P	Average revenue
MR = ΔTR / ΔQ	Marginal revenue
TC = TFC + TVC	Total cost
ATC = TC / Q = AFC + AVC	Average total cost
AFC = TFC / Q	Average fixed cost
AVC = TVC / Q	Average variable cost
MC = ΔTC / ΔQ = ΔTVC / ΔQ	Marginal cost
Profit = TR − TC	Total profit

TR simply multiplies price and quantity. When a firm faces a downward‑sloping demand curve, TR rises, reaches a peak where MR = 0, and then falls. The relationship between TR and MR is tested regularly in WJEC papers.

TR 即价格乘以数量。当企业面对一条向下倾斜的需求曲线时，总收入先上升，在 MR 等于零处达到峰值，然后下降。TR 与 MR 之间的关系在 WJEC 试卷中频繁出现。

AR equals price because every unit is sold at the same market price under perfect competition. In imperfect markets, AR is the demand curve and lies above MR. WJEC candidates must explain why MR lies below AR when the demand curve is downward sloping.

在完全竞争市场，AR 等于价格，因为所有产品按同一市场价格出售。在不完全市场中，AR 即为需求曲线，且位于 MR 上方。考生需解释为什么在需求曲线向下倾斜时 MR 低于 AR。

MC cutting ATC and AVC at their minimum points is a central cost diagram feature. The shutdown point occurs where P = AVC, while the break‑even point is found where P = ATC. Both conditions are frequently examined in relation to short‑run and long‑run decisions.

MC 曲线分别交于 ATC 和 AVC 的最低点，这是成本图示的核心特征。停业点位于 P = AVC 处，盈亏平衡点则处于 P = ATC。这两个条件常结合短期与长期决策进行考察。

Profit maximisation occurs where MC = MR. At this output, the firm cannot increase profit by altering output. WJEC may ask you to calculate profit by finding the difference between AR and ATC at the profit‑maximising quantity, multiplied by Q.

利润最大化发生在 MC = MR 处。在该产量下，企业无法通过改变产量来增加利润。WJEC 可能会要求你用利润最大化产量下的 AR 与 ATC 之差乘以 Q 来计算利润。

3. Market Structure Formulas | 市场结构公式

Different market structures require specific formulas to explain pricing and output decisions. WJEC focuses on perfect competition, monopoly and oligopoly, often asking for numerical illustration of profit conditions.

不同的市场结构需要特定的公式来解释定价和产出决策。WJEC 重点考查完全竞争、垄断和寡头，常要求用数据说明利润条件。

Formula	Context
P = AR = MR = D	Perfect competition
MR = P (1 + 1/η)	Monopoly / imperfect competition (η is PED, negative)
Lerner Index L = (P − MC) / P = −1/η	Measure of market power
n‑firm concentration ratio = (combined sales of top n firms / total market sales) × 100	Oligopoly and market structure

In perfect competition, firms are price‑takers, so the demand curve is perfectly elastic. The firm maximises profit where MC = MR = P. WJEC expects you to identify normal profit, supernormal profit and losses directly from the ATC and AR curves.

完全竞争中，企业是价格接受者，需求曲线完全弹性。企业在 MC = MR = P 处实现利润最大化。WJEC 期望你能直接从 ATC 与 AR 曲线中识别正常利润、超常利润和亏损。

The monopoly pricing formula MR = P(1 + 1/η) links marginal revenue to the price elasticity of demand. Since η is negative, the term (1 + 1/η) is less than 1, so MR < P. This helps explain why a monopolist restricts output to raise price.

垄断定价公式 MR = P(1 + 1/η) 将边际收益与需求的价格弹性联系起来。由于 η 为负数，(1 + 1/η) 小于 1，因此 MR 低于价格 P。这有助于解释垄断者为何限制产量以抬高价格。

The Lerner Index measures the degree of market power: a value closer to 1 indicates stronger monopoly power. WJEC can ask you to calculate the index and comment on its limitations, such as difficulties in measuring marginal cost.

勒纳指数衡量市场势力的程度：数值越接近1，垄断力越强。WJEC 可能会要求计算该指数，并评价其局限性，例如边际成本难以准确衡量。

The n‑firm concentration ratio is commonly used to assess the competitiveness of an oligopolistic market. A high ratio suggests a concentrated market. WJEC often links this to collusive behaviour and pricing strategies.

n 企业集中度比率常用于评估寡头市场的竞争程度。高集中度意味着市场集中。WJEC 常将其与合谋行为和定价策略联系起来。

4. National Income Accounting | 国民收入核算

Macroeconomic performance begins with national income aggregates. WJEC expects you to understand how GDP is measured and to use the circular flow of income.

宏观经济表现始于国民收入总量。WJEC 要求你理解 GDP 的测算方法，并能运用收入循环流动模型。

Formula	Explanation
GDP (expenditure) = C + I + G + (X − M)	Aggregate demand components
GDP (income) = wages + rent + interest + profit	Income method
GDP (output) = value added at each stage of production	Output method
GNI = GDP + net income from abroad	Gross national income

The expenditure method sums consumption (C), investment (I), government spending (G) and net exports (X − M). WJEC often provides data and asks you to calculate GDP by filling gaps in the circular flow. Remember that X − M can be negative, reflecting a trade deficit.

支出法将消费（C）、投资（I）、政府支出（G）和净出口（X − M）相加。WJEC 经常提供数据，要求你填补循环流中的缺口来计算 GDP。注意 X − M 可能为负，反映贸易逆差。

The income method adds up all factor incomes: wages, rent, interest and profits. In the circular flow, the value of income, output and expenditure must be equal. WJEC may ask you to explain why a statistical discrepancy arises in practice.

收入法将工资、租金、利息和利润等所有要素收入加总。在循环流中，收入、产出和支出的价值应相等。WJEC 可能会要求你解释为何现实中存在统计误差。

The output method sums value added at each production stage to avoid double counting. WJEC candidates must distinguish between nominal and real GDP: Real GDP = (Nominal GDP / GDP Deflator) × 100. This adjustment for inflation is essential for meaningful comparisons over time.

产出法将各生产阶段的增加值求和以避免重复计算。考生须区分名义 GDP 和实际 GDP：实际 GDP =（名义 GDP / GDP 平减指数）× 100。这种通胀调整对于进行有意义的跨期比较至关重要。

5. The Multiplier and Propensities | 乘数与边际倾向

The multiplier effect is a cornerstone of WJEC macroeconomic analysis. You must calculate the size of the multiplier and explain how injections affect national income.

乘数效应是 WJEC 宏观经济分析的基石。你必须计算乘数的大小，并解释注入量如何影响国民收入。

Formula	Meaning
MPC = ΔC / ΔY	Marginal propensity to consume
MPS = ΔS / ΔY	Marginal propensity to save
MPT = ΔT / ΔY	Marginal propensity to tax
MPM = ΔM / ΔY	Marginal propensity to import
MPW = MPS + MPT + MPM	Marginal propensity to withdraw
Simple multiplier k = 1 / (1 − MPC) = 1 / MPS	Closed economy, no taxes
Full multiplier k = 1 / MPW	Including taxes and imports
ΔY = k × ΔJ	Change in national income from an injection ΔJ

MPC captures the fraction of additional income spent on domestically produced goods. A high MPC leads to a larger multiplier. WJEC papers often ask you to compute the multiplier from a given MPC and then trace the final change in GDP following a rise in government spending.

MPC 衡量新增收入中用于购买国内生产商品的比例。MPC 越高，乘数越大。WJEC 试卷常要求根据给定的 MPC 计算乘

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A-Level WJEC Economics: Formula Summary Handbook | A-Level WJEC 经济：公式汇总手册