📚 Year 7 Cambridge Economics: Formulas & Theorems Quick Reference | 7年级经济学公式定理速查手册
Welcome to the Year 7 Cambridge Economics Quick Reference Handbook. This guide compiles the essential formulas and theorems you will encounter throughout the course, explained clearly with practical examples. Use it for revision, homework help, or exam preparation.
欢迎使用7年级剑桥经济学速查手册。本指南汇集了你在课程中会遇到的所有基本公式和定理，并通过清晰的解释和实际例子加以说明。可用于复习、作业辅导或考试准备。
1. Opportunity Cost | 机会成本
Opportunity cost is the value of the next best alternative forgone when making a choice. It represents what you give up to obtain something else.
机会成本是指做出选择时放弃的次优选择的价值。它代表你为了得到某物而放弃的东西。
In economics, every choice involves a trade-off. The opportunity cost can be expressed as a simple ratio:
在经济学中，每个选择都涉及取舍。机会成本可以用一个简单的比率来表示：
Opportunity Cost = Sacrifice ÷ Gain
机会成本 = 牺牲 ÷ 获得
For example, if a student chooses to study economics for 2 hours instead of working part-time and earning £20, the opportunity cost of studying is £20.
例如，如果一名学生选择学习经济学2小时，而不是去做兼职赚取20英镑，那么学习的机会成本就是20英镑。
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 given resources and technology. The slope of the PPF is the marginal rate of transformation (MRT).
生产可能性边界显示了一个经济体在给定资源和技术条件下能够生产的两种商品的最大组合。PPF的斜率是边际转换率（MRT）。
MRT = ΔY ÷ ΔX (in absolute value)
MRT = ΔY ÷ ΔX（绝对值）
Points inside the PPF represent inefficient use of resources, while points outside are unattainable. Moving along the PPF illustrates the concept of increasing opportunity cost when the curve is concave.
PPF内部的点代表资源利用不足，而外部的点则无法实现。当PPF曲线凹向原点时，沿着PPF移动体现了机会成本递增的概念。
3. Law of Demand and Demand Function | 需求定律与需求函数
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:
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，反之亦然。一个典型的线性需求函数是：
Qd = a – bP
Qd = a – bP （其中Qd是需求量，P是价格，a和b为正数）
Here ‘a’ represents the quantity demanded when the price is zero (the horizontal intercept), and ‘b’ is the slope showing how much Qd changes per unit change in price.
其中 ‘a’ 表示价格为零时的需求量（水平截距），’b’ 是斜率，表示每单位价格变动引起的需求量变动。
4. Law of Supply and Supply Function | 供给定律与供给函数
The law of supply states that, ceteris paribus, as the price rises, the quantity supplied increases. A simple linear supply function is:
供给定律指出，在其他条件不变的情况下，价格上升，供给量增加。一个简单的线性供给函数是：
Qs = c + dP
Qs = c + dP （c、d为正数）
where ‘c’ is the minimum quantity supplied at zero price (if it exists), and ‘d’ measures the responsiveness of Qs to price changes. When d is larger, supply is more responsive.
其中 ‘c’ 是价格为零时的最低供给量（如果存在），’d’ 衡量供给量对价格变动的反应程度。d 值越大，供给反应越灵敏。
5. Market Equilibrium | 市场均衡
Market equilibrium occurs where the quantity demanded equals the quantity supplied. The equilibrium condition is:
市场均衡发生在需求量等于供给量时。均衡条件是：
Qd = Qs
需求量 = 供给量
To find the equilibrium price, set the demand and supply functions equal and solve for P. Then substitute P back to find equilibrium quantity.
要求得均衡价格，让需求函数等于供给函数，然后求解P。再将P代入任一函数求得均衡数量。
Example: If Qd = 100 – 2P and Qs = –20 + 4P, then 100 – 2P = –20 + 4P → 120 = 6P → P = 20, Q = 60.
示例：如果 Qd = 100 – 2P，Qs = –20 + 4P，那么 100 – 2P = –20 + 4P → 120 = 6P → P = 20，Q = 60。
6. 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动的反应程度。公式为：
PED = (ΔQd ÷ Qd) ÷ (ΔP ÷ P)
需求价格弹性 = (ΔQd ÷ Qd) ÷ (ΔP ÷ P)
Alternatively, using the midpoint method to calculate percentage changes: PED = (ΔQ ÷ average Q) ÷ (ΔP ÷ average P). PED values indicate elastic demand when │PED│ > 1, inelastic when │PED│ < 1, and unitary elastic when │PED│ = 1.
或者，使用中点法计算百分比变化：PED = (ΔQ ÷ 平均Q) ÷ (ΔP ÷ 平均P)。PED 值表示：当│PED│ > 1 时需求富有弹性，│PED│ < 1 时缺乏弹性，│PED│ = 1 时为单位弹性。
7. Price Elasticity of Supply (PES) | 供给价格弹性
Price elasticity of supply measures how much the quantity supplied responds to a change in price. Formula:
供给价格弹性衡量供给量对价格变动的反应程度。公式：
PES = (
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