📚 Year 10 Edexcel Economics: Formula & Theorem Quick Reference Handbook | Edexcel经济：公式定理速查手册
This concise handbook brings together the essential formulas, theorems and key relationships you need to master in Year 10 Edexcel Economics. It covers demand and supply analysis, all major elasticities, cost and revenue structures, break‑even calculations and the multiplier, helping you revise efficiently and apply these tools in exam questions.
这本速查手册汇总了Year 10 Edexcel经济课程中必须掌握的核心公式、定理和关键关系。涵盖需求与供给分析、各大弹性、成本与收益结构、盈亏平衡计算以及乘数效应，帮助你高效复习并在考试中灵活运用这些工具。
1. The Demand Function and Law of Demand | 需求函数与需求定律
The law of demand states that, ceteris paribus, as the price of a good rises, the quantity demanded falls, and vice versa. A typical linear demand function is written as:
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。一个典型的线性需求函数写作：
Qd = a − bP
where 'a' is the intercept (quantity demanded when price is zero) and 'b' is the slope coefficient, showing the fall in Qd for each £1 rise in price. Movements along the demand curve are caused by price changes, while a shift of the whole curve is caused by non‑price factors such as income, tastes or the price of related goods.
其中‘a’为截距（价格为零时的需求量），‘b’为斜率系数，表示价格每上升1英镑所引起的Qd下降量。沿需求曲线的移动由价格变化引起，整条曲线的位移则由收入、偏好或相关商品价格等非价格因素引起。
Law of demand: P↑ → Qd↓, P↓ → Qd↑.
需求定律：P↑ → Qd↓，P↓ → Qd↑。
2. The Supply Function and Law of Supply | 供给函数与供给定律
The law of supply says that, ceteris paribus, a higher price leads to a greater quantity supplied, and a lower price reduces quantity supplied. A simple linear supply function is:
供给定律指出，在其他条件不变的情况下，价格越高，供给量越大；价格越低，供给量越小。简单的线性供给函数为：
Qs = c + dP
where 'c' is the intercept and 'd' is the slope (positive), indicating the change in Qs per unit change in price. Shifts in supply result from changes in costs of production, technology, taxes, subsidies or the number of sellers.
其中‘c’为截距，‘d’为（正）斜率，表示每单位价格变化所引起的供给量变动。供给曲线的位移由生产成本、技术、税收、补贴或卖方数量等因素变化引起。
Law of supply: P↑ → Qs↑, P↓ → Qs↓.
供给定律：P↑ → Qs↑，P↓ → Qs↓。
3. Market Equilibrium | 市场均衡
Market equilibrium occurs where quantity demanded equals quantity supplied. At the equilibrium price (Pe) and quantity (Qe), there is no tendency for change — the market clears.
市场均衡发生在需求量等于供给量之处。在均衡价格（Pe）和均衡数量（Qe）下，市场不存在内在变动趋势——市场出清了。
Qd = Qs at Pe
If price is above Pe, a surplus (excess supply) pushes price down. If price is below Pe, a shortage (excess demand) drives price up. In exams you may be asked to solve for Pe and Qe using simultaneous equations from demand and supply functions.
若价格高于Pe，会出现过剩（超额供给），促使价格下降。若价格低于Pe，会出现短缺（超额需求），推动价格上升。考试中可能要求你利用需求与供给函数联立方程求解Pe和Qe。
4. Price Elasticity of Demand (PED) | 需求价格弹性
PED measures how responsive quantity demanded is to a change in the good’s own price. The formula is:
PED衡量需求量对商品自身价格变化的反应程度。公式为：
PED = %ΔQd / %ΔP
where %Δ means 'percentage change'. The value is typically negative (because demand slopes down), but we often ignore the minus sign in interpretation. Using the midpoint method for more accurate measurement over an arc:
其中%Δ表示‘百分比变化’。该值通常为负（因为需求曲线向下倾斜），但在解释时经常忽略负号。若需更精确地测量弧弹性，可使用中点法：
PED = (ΔQd ÷ Qavg) ÷ (ΔP ÷ Pavg)
The coefficient tells us the type of demand:
弹性系数告诉我们需求类型：
| PED value (ignoring sign) | Type | Meaning |
|---|---|---|
| > 1 | Elastic | %ΔQd > %ΔP |
| = 1 | Unit elastic | %ΔQd = %ΔP |
| < 1 | Inelastic | %ΔQd < %ΔP |
| 0 | Perfectly inelastic | Qd does not change |
| ∞ | Perfectly elastic | Infinite response at a given price |
弹性值（忽略符号） > 1为富有弹性；=1为单位弹性；<1为缺乏弹性；0为完全无弹性；∞为完全弹性。需求量变动百分比大于价格变动百分比表示富有弹性，反之则缺乏弹性。
5. PED and Total Revenue Relationship | 需求价格弹性与总收益关系
Total revenue (TR) is the money a firm receives from sales: TR = P × Q. The relationship between PED and TR is a vital decision‑making tool.
总收益（TR）是企业从销售中获得的总收入：TR = P × Q。PED与TR的关系是一个重要的决策工具。
TR = P × Q
s TR and lowering price reduces TR. If demand is unit elastic (PED = 1), TR stays constant when price changes.
如果需求富有弹性（PED > 1），提价会减少总收益，降价会增加总收益。如果需求缺乏弹性（PED < 1），提价会增加总收益，降价会减少总收益。如果需求具有单位弹性（PED = 1），价格变化时总收益保持不变。
PED > 1: P↓ → TR↑, P↑ → TR↓
富有弹性：降价使TR↑，提价使TR↓
PED < 1: P↓ → TR↓, P↑ → TR↑
缺乏弹性：降价使TR↓，提价使TR↑
PED = 1: TR maximised at that point.
单位弹性：此时TR最大化。
6. Price Elasticity of Supply (PES) | 供给价格弹性
PES measures the responsiveness of quantity supplied to a change in the good’s own price. The formula is:
PES衡量供给量对商品自身价格变化的反应程度。公式为：
PES = %ΔQs / %ΔP
PES is usually positive. Key values:
PES通常为正。关键值：
| PES value | Type |
|---|---|
| > 1 | Elastic supply |
| = 1 | Unit elastic |
| < 1 | Inelastic supply |
| 0 | Perfectly inelastic |
| ∞ | Perfectly elastic |
Determinants of PES include the length of production period, availability of spare capacity, ease of storing stock, and the time period under consideration (momentary, short run, long run).
影响PES的因素包括生产周期的长度、闲置产能的可获得性、库存存储的难易程度以及考虑的时间范围（瞬间、短期、长期）。
7. Income Elasticity of Demand (YED) | 需求收入弹性
YED measures how quantity demanded responds to a change in consumer income. The formula is:
YED衡量需求量对消费者收入变化的反应程度。公式为：
YED = %ΔQd / %ΔY
where Y stands for income. The sign and size classify goods:
其中Y代表收入。符号和大小用来划分商品类型：
YED > 0: normal good (as income rises, demand rises). If YED > 1, it is a luxury good; if 0 < YED < 1, it is a necessity.
YED > 0：正常品（收入上升，需求上升）。若YED > 1，为奢侈品；若0 < YED < 1，为必需品。
YED < 0: inferior good (as income rises, demand falls).
YED < 0：劣等品（收入上升，需求下降）。
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