Year 9 Edexcel Economics: Formula & Theorem Quick Reference Handbook | Year 9 Edexcel 经济：公式定理速查手册

📚 Year 9 Edexcel Economics: Formula & Theorem Quick Reference Handbook | Year 9 Edexcel 经济：公式定理速查手册

This quick reference handbook brings together the essential formulas and economic laws you need to master for the Year 9 Edexcel Economics curriculum. Each concept is explained with clear definitions and real‑world applications to help you revise efficiently.

这本速查手册汇集了Year 9 Edexcel 经济课程所需掌握的核心公式与定理。每个概念都配有清晰的定义和实际应用，帮助你高效复习。

1. The Fundamental Economic Problem: Scarcity, Choice & Opportunity Cost | 基本经济问题：稀缺、选择与机会成本

Economics is the study of how people allocate scarce resources to satisfy unlimited wants. Scarcity forces individuals, firms, and governments to make choices. Whenever a choice is made, the opportunity cost is the value of the next best alternative that must be given up.

经济学研究人们如何分配稀缺资源以满足无限欲望。稀缺性迫使个人、企业和政府做出选择。每当做出选择时，机会成本就是必须放弃的次优选择的价值。

Opportunity cost is a fundamental theorem, not a single equation, but it can be expressed as:

机会成本是一个基本定理，而非单一方程，但可以表示为：

Opportunity Cost = Value of the Next Best Alternative Forgone

Understanding opportunity cost helps decision‑makers weigh trade‑offs, such as whether to study economics or use the time for leisure.

理解机会成本有助于决策者权衡取舍，例如是学习经济还是将时间用于休闲。

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2. Production Possibility Frontier (PPF) | 生产可能性边界

The PPF shows the maximum combinations of two goods that an economy can produce with its existing resources and technology. Points on the curve represent efficient output; points inside indicate underemployment; points outside are unattainable.

生产可能性边界展示了经济体在现有资源和技术下所能生产的两种商品的最大组合。曲线上的点代表有效率产出；曲线内侧的点表示资源未充分利用；曲线外侧的点无法达到。

The slope of the PPF reflects the marginal rate of transformation (MRT) and hence the opportunity cost of producing an additional unit of one good. As production moves along the curve, the law of increasing opportunity cost typically applies, making the PPF concave.

PPF 的斜率反映了边际转换率，因此也反映了多生产一单位某种商品的机会成本。随着生产沿曲线移动，机会成本递增定律通常适用，使得 PPF 呈凹形。

Formula for opportunity cost along the PPF:

沿 PPF 的机会成本公式：

Opportunity Cost of Good X = ΔGood Y ÷ ΔGood X

A shift of the entire PPF outward indicates economic growth, which can result from increases in resources or improvements in technology.

整个 PPF 向外移动表明经济增长，这可能是由于资源增加或技术进步所致。

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3. The Law of Demand | 需求定律

The law of demand states that, ceteris paribus, as the price of a good rises, the quantity demanded falls, and as the price falls, the quantity demanded rises. This inverse relationship is captured by a downward‑sloping demand curve.

需求定律指出，在其他条件不变的情况下，商品价格上升时需求量下降；价格下降时需求量上升。这种反向关系由一条向下倾斜的需求曲线表示。

A simple linear demand function can be written as:

简单的线性需求函数可写为：

Qd = a – bP

where Qd is quantity demanded, P is price, 'a' is the intercept, and 'b' is the slope coefficient. The negative sign embodies the law of demand.

其中 Qd 为需求量，P 为价格，a 为截距，b 为斜率系数。负号体现了需求定律。

Factors that shift the demand curve include changes in income, preferences, prices of related goods (substitutes and complements), expectations, and the number of buyers.

导致需求曲线移动的因素包括收入变化、偏好变化、相关商品（替代品和互补品）价格变化、预期以及买者数量。

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4. The Law of Supply | 供给定律

The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied increases, and as the price falls, the quantity supplied decreases. This positive relationship results in an upward‑sloping supply curve.

供给定律指出，在其他条件不变的情况下，商品价格上升时供给量增加；价格下降时供给量减少。这种正相关关系形成一条向上倾斜的供给曲线。

A typical linear supply function is:

典型的线性供给函数为：

Qs = c + dP

where Qs is quantity supplied, P is price, 'c' is the minimum price at which supply begins, and 'd' is the responsiveness of supply to price changes.

其中 Qs 为供给量，P 为价格，c 为供给开始的最低价格，d 为供给对价格变化的反应程度。

Supply‑curve shifters include input costs, technology, taxes and subsidies, the number of sellers, and expectations about future prices.

供给曲线移动的因素包括投入成本、技术、税收与补贴、卖者数量以及对未来价格的预期。

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5. Market Equilibrium and the Price Mechanism | 市场均衡与价格机制

Market equilibrium occurs where quantity demanded equals quantity supplied. The equilibrium price (Pe) and equilibrium quantity (Qe) are found by solving:

市场均衡发生在需求量等于供给量时。均衡价格（Pe）和均衡数量（Qe）可通过求解下式得到：

Qd = Qs

At any price above equilibrium, a surplus drives the price down; at any price below equilibrium, a shortage drives the price up. This is the invisible hand of the price mechanism that allocates resources.

当价格高于均衡价格时，过剩会推动价格下降；当价格低于均衡价格时，短缺会推动价格上升。这就是价格机制这只“看不见的手”配置资源的方式。

The functions of the price mechanism are signalling, incentive, and rationing. Prices adjust to reflect relative scarcity and guide decisions without central planning.

价格机制的功能包括信号、激励和配给。价格会调整以反映相对稀缺性，无需中央计划即可引导决策。

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

PED 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。公式为：

PED = %ΔQd ÷ %ΔP

Since price and quantity demanded move in opposite directions, PED is usually negative, but economists often refer to its

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