Pre-U WJEC Economics: Quick-Reference Formulas & Theorems | Pre-U WJEC 经济：公式定理速查手册

📚 Pre-U WJEC Economics: Quick-Reference Formulas & Theorems | Pre-U WJEC 经济：公式定理速查手册

This quick-reference handbook compiles the essential formulas and theorems you need to master for the Pre-U WJEC Economics syllabus. Use it as a portable revision tool to reinforce your understanding of quantitative relationships, key definitions and the conditions that underpin micro and macro analysis.

本速查手册汇编了 Pre-U WJEC 经济学课程中必须掌握的核心公式与定理。你可以将其用作便携复习工具，以加深对关键数量关系、重要定义以及支撑微观与宏观分析之条件的理解。

1. Price Elasticity of Demand (PED) | 需求价格弹性 (PED)

Price elasticity of demand measures how responsive the quantity demanded of a good is to a change in its own price, ceteris paribus.

需求价格弹性衡量在其他条件不变的情况下，一种商品的需求量对其自身价格变动的反应程度。

The formula is: PED = %ΔQd / %ΔP = (ΔQd / Qd) ÷ (ΔP / P). A coefficient of −2 (ignoring sign) means a 1% price rise causes a 2% fall in quantity demanded.

计算公式为：PED = %ΔQd / %ΔP = (ΔQd / Qd) ÷ (ΔP / P)。若系数为 −2（忽略符号），则表示价格上涨 1% 将导致需求量下降 2%。

Demand is price elastic when |PED| > 1, unit elastic when |PED| = 1, and price inelastic when |PED| < 1. A horizontal demand curve indicates perfect elasticity; a vertical curve indicates perfect inelasticity.

当 |PED| > 1 时需求富有弹性；|PED| = 1 时为单位弹性；|PED| < 1 时需求缺乏弹性。水平需求曲线表示完全弹性，垂直需求曲线表示完全无弹性。

2. Income Elasticity of Demand (YED) | 需求收入弹性 (YED)

Income elasticity of demand captures the responsiveness of demand to a change in consumers’ real income.

需求收入弹性反映需求量对消费者实际收入变动的反应程度。

It is calculated as: YED = %ΔQd / %ΔY, where Y stands for real income. Luxury goods have YED > 1; necessities have 0 < YED < 1; inferior goods have YED < 0.

计算公式为：YED = %ΔQd / %ΔY，其中 Y 表示实际收入。奢侈品的 YED > 1；必需品的 YED 介于 0 与 1 之间；劣质品的 YED < 0。

A positive YED identifies a normal good, and a negative YED identifies an inferior good. Firms use YED to forecast sales as incomes change through the business cycle.

YED 为正意味着该商品是正常品，YED 为负则为劣质品。企业利用 YED 预测在商业周期中收入变化对销售的影响。

3. Cross-Price Elasticity of Demand (XED) | 需求交叉价格弹性 (XED)

Cross-price elasticity of demand measures how the quantity demanded of one good, say A, responds to a price change in another good, B, all other factors held constant.

需求交叉价格弹性衡量在其他因素不变的情况下，商品 A 的需求量对商品 B 价格变动的反应程度。

It is given by: XED = %ΔQd of good A / %ΔP of good B. A positive XED signals substitutes (e.g. tea for coffee); a negative XED signals complements (e.g. printers and ink).

计算公式为：XED = %ΔQd of good A / %ΔP of good B。XED 为正表明两者为替代品（如茶与咖啡）；XED 为负表明两者为互补品（如打印机与墨水）。

The larger the absolute value of XED, the stronger the relationship between the two goods. A value close to zero implies independent goods.

XED 的绝对值越大，两商品之间关系越强。XED 接近零表明两者为独立品。

4. Price Elasticity of Supply (PES) | 供给价格弹性 (PES)

Price elasticity of supply quantifies how the quantity supplied of a good changes in response to a change in its own price.

供给价格弹性衡量一种商品的供给量对其自身价格变动的反应程度。

The formula is: PES = %ΔQs / %ΔP = (ΔQs / Qs) ÷ (ΔP / P). Supply is price elastic when PES > 1, inelastic when PES < 1, and unit elastic when PES = 1.

计算公式为：PES = %ΔQs / %ΔP = (ΔQs / Qs) ÷ (ΔP / P)。当 PES > 1 时供给富有弹性；PES < 1 时缺乏弹性；PES = 1 时为单位弹性。

Key determinants include the availability of spare capacity, the length of the production period, the ability to store stocks, and the time frame considered.

影响供给弹性的主要因素包括闲置产能的大小、生产周期长短、库存能力以及所考虑的时间跨度。

5. Cost Measures | 成本度量

Firms face a variety of cost concepts that underpin pricing and output decisions. Total cost (TC) is the sum of total fixed cost (TFC) and total variable cost (TVC): TC = TFC + TVC.

企业面临多种成本概念，它们是定价与产量决策的基础。总成本 (TC) 是总固定

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