📚 Year 9 Edexcel Economics: Formula & Theorem Quick Reference Handbook | Year 9 Edexcel 经济：公式定理速查手册
Welcome to your go-to quick reference handbook for Year 9 Edexcel Economics. This guide brings together all the essential formulas, theorems and key definitions you need to master the basics of economic analysis. From scarcity and opportunity cost to elasticity, costs and profit, each concept is presented with a clear statement, relevant formula and practical interpretation. Use this handbook to consolidate your revision, test your understanding and build a solid foundation for higher-level economics.
欢迎使用这本 Year 9 Edexcel 经济学速查手册。本书聚合了你需要掌握的所有基本公式、定理和关键定义，涵盖从稀缺性和机会成本到弹性、成本与利润等主题。每个概念都以清晰的陈述、相关公式和实用的解读方式呈现。使用本手册来巩固复习、自测理解，并为更高级的经济学学习打下坚实基础。
1. Scarcity, Choice and Opportunity Cost Theorem | 稀缺性、选择与机会成本定理
Scarcity is the fundamental economic problem that arises because resources are limited while human wants are unlimited. This forces all economic agents to make choices about how to allocate resources.
稀缺性是由有限资源与无限欲望之间的对立而产生的根本经济问题。这迫使所有经济主体就如何配置资源做出选择。
Opportunity cost is the value of the next best alternative that is forgone whenever a choice is made. It is not simply the money spent, but the benefit lost from the option not taken.
机会成本是在做出选择时所放弃的次优选项的价值。它不仅仅是花掉的钱，而是没有选择的那项替代行动所带来的收益损失。
The theorem of opportunity cost states that every decision carries a cost hidden in the alternatives not pursued. Rational choices aim to minimise opportunity cost.
机会成本定理指出，每一个决策都隐含着未选择的替代方案所构成的成本。理性的选择旨在最小化机会成本。
2. Law of Demand and Demand Equation | 需求定律与需求公式
The law of demand states that, ceteris paribus (all other factors held constant), as the price of a good rises, the quantity demanded of that good falls, and vice versa.
需求定律指出，在其他条件不变的前提下，一种商品的价格上升，其需求量下降；价格下降，需求量上升。
A linear demand function can be written as:
Qd = a – bP
where Qd is quantity demanded, P is price, a is the intercept (quantity demanded when price is zero) and b is the slope coefficient showing the responsiveness of quantity to price changes.
其中 Qd 为需求量，P 为价格，a 为截距（价格为零时的需求量），b 为斜率系数，表示需求量对价格变化的反应程度。
A negative b confirms the inverse relationship between price and quantity demanded. A higher value of b means demand is more responsive to price changes.
b 为负值证实了价格与需求量之间的反向关系。b 值越大，需求对价格变化的反应越敏感。
3. Law of Supply and Supply Equation | 供给定律与供给公式
The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied of that good increases. Producers are willing to supply more when they can obtain a higher price.
供给定律指出，在其他条件不变的前提下，一种商品的价格上升，其供给量增加。当能获得更高价格时，生产者愿意提供更多产品。
A linear supply function can be written as:
Qs = c + dP
where Qs is quantity supplied, P is price, c is the intercept (often representing minimum price needed to supply anything) and d is the slope coefficient showing the relationship between price and quantity supplied.
其中 Qs 为供给量，P 为价格，c 为截距（通常代表开始供给所需的最低价格），d 为斜率系数，表示价格与供给量之间的正比关系。
A positive d indicates that price and quantity supplied move in the same direction. The steeper the supply curve, the less responsive supply is to price changes.
d 为正值表明价格与供给量同向变动。供给曲线越陡峭，供给对价格变化的反应越不敏感。
4. Market Equilibrium | 市场均衡
demanded equals quantity supplied: Qd = Qs. At this price, there is no tendency for change – the market clears.
市场均衡发生在需求量等于供给量的价格水平上：Qd = Qs。在该价格水平下，市场出清，没有自行变动的趋势。
If the market price is above equilibrium, there is excess supply (a surplus), putting downward pressure on price. If the price is below equilibrium, there is excess demand (a shortage), pushing prices upward.
若市场价格高于均衡价格，就会出现超额供给（过剩），对价格产生下行压力。若价格低于均衡价格，则出现超额需求（短缺），推动价格上行。
By solving the demand and supply equations simultaneously, the equilibrium price and quantity can be found:
Equilibrium condition: Qd = Qs
For example, if Qd = 100 – 2P and Qs = –20 + 4P, setting them equal yields 100 – 2P = –20 + 4P → 120 = 6P → P = 20, Q = 60.
通过联立需求和供给方程，可以求出均衡价格与均衡数量。例如，若 Qd = 100 – 2P 且 Qs = –20 + 4P，令两者相等可得 P = 20，Q = 60。
5. Price Elasticity of Demand (PED) | 需求价格弹性
Price elasticity of demand measures how responsive the quantity demanded is to a change in price. It is calculated as:
PED = (% Change in Quantity Demanded) / (% Change in Price)
or using the midpoint formula for accuracy over a segment:
PED = (ΔQd / Qd_avg) / (ΔP / Pavg)
需求价格弹性衡量需求量对价格变化的反应程度。它被计算为：需求量变化百分比除以价格变化百分比。为了精确，可运用弧弹性中点公式。
The numerical value of PED is usually negative (reflecting the law of demand), but by convention we often use the absolute value. Key categories are:
| |PED| > 1 | Elastic demand – quantity changes proportionately more than price. |
| |PED| < 1 | Inelastic demand – quantity changes proportionately less than price. |
| |PED| = 1 | Unitary elastic – quantity and price change by the same percentage. |
| |PED| = 0 | Perfectly inelastic – quantity demanded does not change as price varies. |
| |PED| = ∞ | Perfectly elastic – any price rise above a certain level causes demand to fall to zero. |
通常 PED 为负值（反映需求定律），但我们习惯使用绝对值。主要分类包括：|PED| > 1 富有弹性；|PED| < 1 缺乏弹性；|PED| = 1 单位弹性；|PED| = 0 完全无弹性；无限大则为完全有弹性。
6. Price Elasticity of Supply (PES) | 供给价格弹性
Price elasticity of supply measures the responsiveness of quantity supplied to a change in price. It is calculated as:
PES = (% Change in Quantity Supplied) / (% Change in Price)
PES is usually positive because of the law of supply. Key interpretations:
| PES > 1 | Elastic supply – producers can increase output easily when price rises. |
| PES < 1 | Inelastic supply – output is less responsive; often due to time lags or limited capacity. |
| PES = 0 | Perfectly inelastic – quantity supplied is fixed regardless of price. |
供给价格弹性衡量供给量对价格变化的反应程度。PES通常为正数。主要解读：PES > 1 富有弹性；PES < 1 缺乏弹性；PES = 0 完全无弹性。
The main determinant of PES is the time period: supply is more elastic in the long run as producers have time to adjust capacity.
决定 PES 的主要因素是时间周期：长期中生产者有时长调整产能，供给更具弹性。
7. Income Elasticity of Demand (YED) | 需求收入弹性
Income elasticity of demand measures how the quantity demanded responds to a change in consumers' income (Y). The formula is:
YED = (% Change in Quantity Demanded) / (% Change in Income)
The sign and magnitude of YED indicate the type of good:
Normal goods have a positive YED. If YED > 1, the good is a luxury; if 0 < YED < 1, it is a necessity.
正常商品的 YED 为正。YED > 1 为奢侈品；0 < YED < 1 为必需品。
Inferior goods have a negative YED: demand falls as income rises.
劣质商品的 YED 为负：收入上升时需求下降。
需求收入弹性衡量需求量对消费者收入变化的反应。YED 的正负和大小可用来区分商品类型。
8. Cross Elasticity of Demand (XED) | 需求交叉弹性
Cross elasticity of demand measures the responsiveness of demand for one good (A) when the price of another good (B) changes. The formula is:
XED = (% Change in Quantity Demanded of Good A) / (% Change in Price of Good B)
The sign of XED reveals the relationship between the two goods:
XED > 0: Substitutes – an increase in the price of B raises demand for A (e.g. tea and coffee).
XED > 0：替代品 – B 的价格上升会增加对 A 的需求（如茶和咖啡）。
XED < 0: Complements – an increase in the price of B reduces demand for A (e.g. printers and ink).
XED < 0：互补品 – B 的价格上升会减少对 A 的需求（如打印机和墨盒）。
XED = 0: Unrelated goods – no relationship.
XED = 0：无关商品 – 没有关联。
需求交叉弹性衡量一种商品的需求量对另一种商品价格变化的反应。XED 的正负揭示了商品间的替代或互补关系。
9. Cost Formulas | 成本公式
Cost analysis is central to a firm's production decisions. The key cost concepts and formulas include:
成本分析是企业生产决策的核心。关键成本概念及公式包括：
Total Cost (TC) = Total Fixed Cost (TFC) + Total Variable Cost (TVC)
总成本 (TC) = 总固定成本 (TFC) + 总可变成本 (TVC)
TC = TFC + TVC
Average Total Cost (ATC) = TC / Quantity (Q). It shows the cost per unit of output.
平均总成本 (ATC) = TC / 产量 (Q)。它表示单位产出的成本。
ATC = TC / Q
Similarly, Average Fixed Cost (AFC) = TFC / Q and Average Variable Cost (AVC) = TVC / Q.
同理，平均固定成本 (AFC) = TFC / Q；平均可变成本 (AVC) = TVC / Q。
l Cost (MC) is the extra cost of producing one more unit: MC = Change in TC / Change in Q.
边际成本 (MC) 是额外生产一单位所带来的总成本增加量：MC = ΔTC / ΔQ。
MC = ΔTC / ΔQ
The law of diminishing returns states that adding more of a variable input to a fixed input eventually yields lower marginal gains, causing MC to rise after a certain point.
边际报酬递减定律指出，在固定投入上持续增加可变投入，最终会导致边际产出下降，从而在某一拐点后边际成本上升。
10. Revenue and Profit Formulas | 收入与利润公式
Revenue is the income a firm receives from selling its output. The fundamental formulas are:
Total Revenue (TR) = Price (P) × Quantity (Q)
ue (AR) = TR / Q = P. Thus, the demand curve is also the AR curve for a price-taking firm.
总收入 (TR) = 价格 (P) × 数量 (Q)。平均收入 (AR) = TR / Q = P。因此，对于价格接受者，需求曲线也是平均收入曲线。
Profit is the reward for risk-taking and is calculated as the difference between total revenue and total cost.
利润是对承担风险的回报，其计算公式为总收入与总成本之差。
Profit (π) = TR – TC
gin measures profitability as a percentage of revenue: Profit Margin = (Profit / TR) × 100%.
利润率衡量的是利润占收入的百分比：利润率 = (利润 / TR) × 100%。
A firm's objective is often profit maximisation, which occurs where marginal revenue (MR) equals marginal cost (MC). At this point, the extra revenue from selling one more unit exactly covers the extra cost of producing it.
企业往往以利润最大化为目标，这发生在边际收入 (MR) 等于边际成本 (MC) 之处。在该点，多销售一单位所带来的额外收入恰好等于生产该单位的额外成本。
11. Law of Diminishing Marginal Utility | 边际效用递减定律
Marginal utility is the additional satisfaction gained from consuming one more unit of a good. The law of diminishing marginal utility states that as a person consumes more units of a good, the extra satisfaction from each additional unit eventually decreases.
边际效用是指额外消费一单位商品所带来的满足感增量。边际效用递减定律指出，随着个人消费更多的同种商品，每额外一单位商品带来的满足感增量最终会下降。
This law helps explain why the demand curve slopes downward: consumers are willing to pay less for additional units because they obtain less utility from them.
这一定律有助于解释需求曲线向下倾斜的原因：消费者因额外单位带来的效用减少而不愿支付高价。
Mathematically, total utility may increase at a decreasing rate, while marginal utility falls. Eventually, marginal utility can become negative.
数学上，总效用可能以递减的速度增加，而边际效用不断下降，甚全可变为负数。
Understanding this law is essential for comprehending consumer behaviour and pricing strategies. It also underpins the concept of consumer surplus.
理解该定律对于把握消费者行为和定价策略至关重要，同时也为消费者剩余概念奠定了基础。
Published by TutorHao | Economics Revision Series | aleveler.com
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