📚 Year 11 CAIE Economics: Formula & Theorem Quick Reference Handbook | 剑桥IGCSE经济公式定理速查手册
This quick reference handbook compiles the essential formulas, theorems, and key principles required for the CAIE IGCSE Economics (Year 11) syllabus. It is designed to help you revise efficiently and apply economic concepts with confidence in both multiple-choice and structured questions.
本速查手册汇编了剑桥IGCSE经济（11年级）课程所必需的核心公式、定理与关键原理，旨在帮助你高效复习，在选择题与结构化问题中自信运用经济学概念。
1. The Law of Demand and Law of Supply | 需求定律与供给定律
The law of demand states that, ceteris paribus, when the price of a good rises, the quantity demanded falls, and vice versa.
需求定律指出，在其他条件不变的情况下，商品价格上升，需求量下降；反之亦然。
The law of supply states that, ceteris paribus, when the price of a good rises, the quantity supplied increases, and when the price falls, the quantity supplied decreases.
供给定律指出，在其他条件不变的情况下，商品价格上升，供给量增加；价格下降，供给量减少。
Market equilibrium occurs where the demand curve intersects the supply curve. At this point, the price equates quantity demanded and quantity supplied:
市场均衡发生在需求曲线与供给曲线的交点处，此时价格使需求量等于供给量：
Qd = Qs
Any price above equilibrium creates a surplus (excess supply), while a price below equilibrium creates a shortage (excess demand).
任何高于均衡的价格都会产生过剩（超额供给），而低于均衡的价格则会产生短缺（超额需求）。
2. Price Elasticity of Demand (PED) | 需求价格弹性
Price elasticity of demand (PED) measures the responsiveness of quantity demanded to changes in price. It is calculated as:
需求价格弹性（PED）衡量需求量对价格变化的反应程度，计算公式为：
PED = (%Δ Quantity Demanded) / (%Δ Price)
If |PED| > 1, demand is price elastic. If |PED| < 1, demand is price inelastic. If |PED| = 1, demand is unit elastic. If |PED| = 0, demand is perfectly inelastic. If |PED| → ∞, demand is perfectly elastic.
如果 |PED| > 1，需求富有价格弹性；|PED| < 1 则缺乏价格弹性；|PED| = 1 为单位弹性；|PED| = 0 为完全无弹性；|PED| → ∞ 为完全弹性。
Key determinants of PED include the availability of substitutes, the degree of necessity, the proportion of income spent on the good, and the time period under consideration.
PED 的主要决定因素包括：替代品的可获得性、必需程度、支出在收入中所占的比重以及所考虑的时间周期。
| Determinant (决定因素) | Effect on PED (对PED的影响) |
|---|---|
| Number of substitutes (替代品数量) | More substitutes → demand more elastic (更多替代品 → 需求更富弹性) |
| Degree of necessity (必需程度) | Necessities → inelastic; luxuries → elastic (必需品 → 缺乏弹性；奢侈品 → 富有弹性) |
| Proportion of income (收入比重) | Larger proportion → more elastic (比重越大 → 越富弹性) |
| Time period (时间周期) | Longer period → more elastic (时间越长 → 越富弹性) |
3. Price Elasticity of Supply (PES) | 供给价格弹性
Price elasticity of supply (PES) measures the responsiveness of quantity supplied to a change in price. The formula is:
供给价格弹性（PES）衡量供给量对价格变化的反应程度，计算公式为：
PES = (%Δ Quantity Supplied) / (%Δ Price)
ly is unit elastic. If PES = 0, supply is perfectly inelastic (e.g. fixed supply of land).
如果 PES > 1，供给富有价格弹性；PES < 1 则缺乏弹性；PES = 1 为单位弹性；PES = 0 为完全无弹性（如土地的固定供给）。
PES is heavily influenced by the time period and the ease with which firms can increase output. In the short run, supply tends to be more inelastic; in the long run, it becomes more elastic.
PES 主要受时间周期和厂商增加产量的难易程度影响。短期中供给往往较为缺乏弹性，长期中则变得更有弹性。
4. Income Elasticity of Demand (YED) | 需求收入弹性
Income elasticity of demand (YED) measures the responsiveness of quantity demanded to a change in income. It is calculated as:
需求收入弹性（YED）衡量需求量对收入变化的反应程度，计算公式为：
YED = (%Δ Quantity Demanded) / (%Δ Income)
A positive YED indicates a normal good; within that, YED > 1 suggests a luxury good, while 0 < YED < 1 suggests a necessity. A negative YED indicates an inferior good, meaning demand falls as income rises.
YED 为正表示正常商品；其中 YED > 1 为奢侈品，0 < YED < 1 为必需品。YED 为负表示劣等品，即收入增加时需求反而下降。
| YED Value (YED值) | Type of Good (商品类型) |
|---|---|
| Positive, > 1 (正值, >1) | Luxury good (奢侈品) |
| Positive, 0 to 1 (正值, 0至1) | Normal necessity (正常必需品) |
| Negative (负值) | Inferior good (劣等品) |
5. Cross Elasticity of Demand (XED) | 需求交叉弹性
Cross elasticity of demand (XED) measures the responsiveness of quantity demanded for one good (A) to a change in the price of another good (B). The formula is:
需求交叉弹性（XED）衡量一种商品(A)的需求量对另一种商品(B)价格变化的反应程度，公式为：
XED = (%Δ Quantity Demanded of A) / (%Δ Price of B)
A positive XED indicates that the two goods are substitutes (e.g. tea and coffee). A negative XED indicates that the goods are complements (e.g. printers and ink cartridges). An XED close to zero suggests the goods are unrelated.
XED 为正表示两种商品互为替代品（如茶与咖啡）；XED 为负表示两种商品为互补品（如打印机与墨盒）；XED 接近于零则表明商品间无相关关系。
6. Cost Formulas | 成本公式
Understanding a firm's cost structure is fundamental to analysing production decisions. The following formulas are used to calculate various cost measures in the short run.
理解企业的成本结构是分析生产决策的基础。以下公式用于计算短期内的各种成本指标。
Total cost (TC) is the sum of total fixed cost (TFC) and total variable cost (TVC):
总成本 (TC) 是总固定成本 (TFC) 与总可变成本 (TVC) 之和：
TC = TFC + TVC
Average total cost (ATC) is total cost divided by output (Q):
平均总成本 (ATC) 是总成本除以产量 (Q)：
ATC = TC / Q
Average variable cost (AVC) and average fixed cost (AFC) are found as follows:
平均可变成本 (AVC) 和平均固定成本 (AFC) 计算公式如下：
AVC = TVC / Q
AFC = TFC / Q
Marginal cost (MC) is the additional cost of producing one more unit of output:
边际成本 (MC) 是每多生产一单位产量所增加的成本：
MC = ΔTC / ΔQ
Fixed costs do not vary with output, while variable costs change as output changes. The ATC curve is typically U-shaped due to the law of diminishing returns.
固定成本不随产量变化，可变成本随产量变化。由于边际收益递减规律，ATC 曲线通常呈U形。
7. Revenue and Profit Maximisation | 收益与利润最大化
Total revenue (TR) is the income a firm receives from selling its output. It is calculated as price multiplied by quantity sold:
总收益 (TR) 是企业从销售产品中获得的收入，计算公式为价格乘以销量：
TR = P × Q
Average revenue (AR) is total revenue per unit sold, which is equal to the price of the good:
平均收益 (AR) 是每单位销售的收益，等于商品的价格：
AR = TR / Q = P
Marginal revenue (MR) is the additional revenue from selling one more unit:
边际收益 (MR) 是每多销售一单位所带来的额外收益：
MR = ΔTR / ΔQ
als marginal cost, i.e. MR = MC. Providing that MC is rising at that point, the firm maximises profit.
利润 (π) 是总收益与总成本之间的差额。任何企业实现利润最大化的条件均为边际收益等于边际成本，即 MR = MC。若在该点 MC 处于上升阶段，企业即实现利润最大化
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