📚 Year 13 CAIE Economics: Formula & Theorem Quick Reference Handbook | Year 13 CAIE 经济：公式定理速查手册
This article provides a concise revision reference for Year 13 CAIE Economics students, covering the essential formulas and theorems required for A2 examinations. Each section presents the core mathematical relationships along with clear explanations and contextual applications, from elasticity calculations to multiplier theory and cost structures.
本文为 Year 13 CAIE 经济学生提供简洁的复习参考，涵盖 A2 考试必备的核心公式与定理。每个部分都呈现核心数学关系，并附有清晰的解释与实际应用背景，涵盖弹性计算、乘数理论及成本结构等。
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. The formula is:
需求价格弹性衡量需求量对价格变化的反应程度。公式如下：
PED = (%ΔQd) ÷ (%ΔP)
Since price and quantity demanded are inversely related, the value of PED is usually negative, but economists often refer to its absolute value. Demand is price elastic when |PED| > 1, price inelastic when |PED| < 1, and unit elastic when |PED| = 1. The magnitude of PED determines the impact of a price change on total revenue: if demand is elastic, a price cut raises total revenue; if demand is inelastic, a price rise raises total revenue.
由于价格与需求量反向变动，PED 通常为负值，但经济学家常指其绝对值。当 |PED| > 1 时需求富有弹性，|PED| < 1 时需求缺乏弹性，|PED| = 1 时为单位弹性。PED 的大小决定了价格变化对总收益的影响：需求富有弹性时，降价增加总收益；需求缺乏弹性时，提价增加总收益。
Factors influencing PED include the number and closeness of substitutes, the proportion of income spent on the good, whether it is a necessity or a luxury, and the time allowed for adjustment. Goods with many close substitutes typically have more elastic demand.
影响 PED 的因素包括替代品的数量及相近程度、商品在收入中所占比重、是必需品还是奢侈品，以及允许调整的时间周期。拥有众多相近替代品的商品，其需求往往更富弹性。
2. Income Elasticity of Demand (YED) | 需求收入弹性
Income elasticity of demand indicates how quantity demanded responds to changes in consumer income. The formula is:
需求收入弹性表明需求量如何随消费者收入变动。公式为：
YED = (%ΔQd) ÷ (%ΔY)
Y represents income. A positive YED signals a normal good: it is a necessity if 0 < YED < 1, and a luxury if YED > 1. A negative YED identifies an inferior good, where demand falls as income rises. The coefficient helps firms predict how sales will react over the business cycle and can inform decisions on product portfolios.
Y 代表收入。YED 为正意味着正常品：若 0 < YED < 1 为必需品，若 YED > 1 为奢侈品。YED 为负则为低档品，收入增加时需求下降。该系数有助于企业预测销售如何随经济周期变化，并为产品组合决策提供依据。
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