📚 KS3 AQA Economics: Formula and Theorem Quick Reference | KS3 AQA 经济：公式定理速查手册
Economics is often called the science of choice. At KS3 level, building a strong foundation requires understanding the fundamental formulas and economic laws that explain how people, businesses, and governments make decisions. This quick-reference guide brings together the essential formulas and theorems in one place, with each concept explained clearly in both English and Chinese to support bilingual learners.
经济学常被称为选择的科学。在 KS3 阶段，要打好坚实基础，就必须理解那些解释个人、企业和政府如何决策的基本公式与经济规律。这份速查手册将核心公式和定理汇集在一起，每个概念都用英文和中文清晰解释，为双语学习者提供支持。
1. Scarcity and Choice | 稀缺性与选择
Scarcity means that resources are limited while human wants are unlimited. This forces individuals and societies to make choices about how to allocate resources.
稀缺性意味着资源有限而人类的欲望无限。这迫使个人和社会必须就如何配置资源做出选择。
The basic economic problem can be summarised as: what to produce, how to produce, and for whom to produce.
基本经济问题可以概括为：生产什么、如何生产以及为谁生产。
2. Opportunity Cost | 机会成本
Opportunity cost is the value of the next best alternative that is given up when a choice is made. It is not simply the money spent, but what you could have gained from the next best option.
机会成本是指做出一种选择时所放弃的次优选择的价值。它不仅仅是指花费的金钱，而是你本可以从次优选项中获得的收益。
Opportunity Cost = Value of Next Best Alternative Sacrificed
机会成本 = 所放弃的次优选择的价值
For example, if a student spends one hour studying economics instead of playing football, the opportunity cost is the enjoyment and health benefit of that football game.
例如，如果一名学生花一小时学习经济而不是踢足球，那么机会成本就是那场足球赛带来的乐趣和健康收益。
3. Demand and the Law of Demand | 需求与需求定律
Demand is the quantity of a good or service that consumers are willing and able to buy at a given price over a period of time. The law of demand states that, ceteris paribus (all else being equal), as the price of a good rises, the quantity demanded falls, and vice versa.
需求是指在一段时间内，消费者愿意且能够以某一价格购买的商品或服务的数量。需求定律指出，在其他条件不变的情况下，当商品价格上升时，需求量下降，反之亦然。
P ↑ → Qd ↓ and P ↓ → Qd ↑
价格上升 → 需求量下降；价格下降 → 需求量上升
This inverse relationship is represented by a downward-sloping demand curve.
这种反向关系表现为一条向下倾斜的需求曲线。
4. Supply and the Law of Supply | 供给与供给定律
Supply is the quantity of a good or service that producers are willing and able to offer for sale at a given price. The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied increases, and as price falls, quantity supplied falls.
供给是指生产者在某一价格下愿意且能够出售的商品或服务的数量。供给定律指出，在其他条件不变的情况下，当商品价格上升时，供给量增加；价格下降时，供给量减少。
P ↑ → Qs ↑ and P ↓ → Qs ↓
价格上升 → 供给量上升；价格下降 → 供给量下降
Producers are motivated by profit, so higher prices give them an incentive to produce more.
生产者受利润驱动，因此更高的价格激励他们增加生产。
5. Market Equilibrium | 市场均衡
Market equilibrium occurs where the demand curve and supply curve intersect. At this point, the quantity demanded equals the quantity supplied. There is no shortage or surplus.
市场均衡发生在需求曲线与供给曲线相交之处。在该点，需求量等于供给量，既不存在短缺也不存在过剩。
Equilibrium: Qd = Qs
均衡条件：需求量 = 供给量
If the market price is above the equilibrium, there will be a surplus (excess supply), putting downward pressure on price. If below, a shortage (excess demand) will push the price up.
若市场价格高于均衡价格，就会出现过剩（超额供给），产生价格下行压力；若低于均衡价格，则会出现短缺（超额需求），推动价格上涨。
6. Price Elasticity of Demand | 需求价格弹性
Price elasticity of demand (PED) measures how responsive the quantity demanded is to a change in price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price.
需求价格弹性（PED）衡量需求量对价格变化的反应程度。它等于需求量变动的百分比除以价格变动的百分比。
PED = %Δ Qd ÷ %Δ P
需求价格弹性 = 需求量变动百分比 ÷ 价格变动百分比
- If PED > 1, demand is elastic (sensitive to price changes).
- If PED < 1, demand is inelastic (not very sensitive).
- If PED = 1, demand is unit elastic.
- 如果 PED > 1，需求富有弹性（对价格变化敏感）。
- 如果 PED < 1，需求缺乏弹性（不太敏感）。
- 如果 PED = 1，需求具有单位弹性。
7. Costs of Production | 生产成本
Total cost (TC) is the sum of fixed costs (FC) and variable costs (VC). Fixed costs do not change with output (e.g. rent), while variable costs do (e.g. raw materials).
总成本（TC）是固定成本（FC）与可变成本（VC）之和。固定成本不随产量变化（如租金），而可变成本会变化（如原材料）。
Total Cost = Fixed Cost + Variable Cost
TC = FC + VC
Average cost (AC) is the cost per unit of output, found by dividing total cost by quantity.
平均成本（AC）是每单位产量的成本，由总成本除以数量求得。
Average Cost = TC ÷ Q
平均成本 = 总成本 ÷ 数量
8. Revenue and Profit | 收入与利润
Total revenue (TR) is the money received from selling goods or services. It is calculated by multiplying the price per unit by the number of units sold.
总收入（TR）是销售商品或服务所获得的款项，由单价乘以销售数量计算。
Total Revenue = Price × Quantity
TR = P × Q
Profit is what remains after all costs are deducted from revenue.
利润是从收入中扣除所有成本后的剩余部分。
Profit = Total Revenue − Total Cost
利润 = 总收入 − 总成本
If total cost exceeds total revenue, the business makes a loss.
如果总成本超过总收入，企业就发生亏损。
9. Economic Systems | 经济体制
An economic system determines how resources are allocated. The three main types are market economy, planned (command) economy, and mixed economy.
经济体制决定资源如何配置。三种主要类型是市场经济、计划（指令）经济和混合经济。
- Market economy: resources allocated by price mechanism (supply and demand).
- Planned economy: government decides allocation.
- Mixed economy: combination of both, with private and public sectors.
- 市场经济：通过价格机制（供给与需求）配置资源。
- 计划经济：政府决定资源配置。
- 混合经济：两者结合，私营与公共部门并存。
There is no single formula, but the key concept is how these systems answer the three basic economic questions.
这里没有单一公式，但关键概念在于这些体制如何回答三个基本经济问题。
10. Money and Inflation | 货币与通货膨胀
Inflation is a sustained rise in the general price level of goods and services over time. It reduces the purchasing power of money.
通货膨胀是指商品和服务的总体价格水平在一段时期内持续上涨。它降低了货币的购买力。
Inflation is often measured using the Consumer Price Index (CPI). The simple percentage change formula for inflation is:
通货膨胀通常用消费者价格指数（CPI）衡量。计算通胀率的简单百分比变化公式为：
Inflation Rate = [(Price Index₂ − Price Index₁) ÷ Price Index₁] × 100 %
通胀率 = [(价格指数₂ − 价格指数₁) ÷ 价格指数₁] × 100%
Central banks aim to keep inflation low and stable, often around 2%.
中央银行的目标是将通胀保持在低位且稳定，通常为 2% 左右。
11. Specialisation and Trade | 专业化与贸易
Specialisation occurs when individuals, businesses, or countries focus on producing a limited range of goods or services. It increases efficiency and output.
专业化发生在个人、企业或国家集中生产有限种类的商品或服务时。它能提高效率和产量。
The theory of comparative advantage suggests that countries should specialise in producing goods where they have a lower opportunity cost, and then trade with others to obtain goods they produce less efficiently.
比较优势理论指出，各国应专业化生产机会成本较低的商品，然后通过贸易获得其生产效率较低的商品。
This leads to mutual gains from trade, enabling higher overall consumption.
这带来贸易互惠，使总体消费水平更高。
12. Externalities | 外部性
An externality is a cost or benefit that affects a third party who did not choose to incur that cost or benefit. Negative externalities (e.g. pollution) impose costs on others, while positive externalities (e.g. education) provide benefits.
外部性是指影响到未选择承担该成本或收益的第三方的成本或收益。负外部性（如污染）给他人带来成本，而正外部性（如教育）则为他人带来收益。
Governments may intervene to correct externalities using taxes (for negative externalities) or subsidies (for positive externalities). This is known as internalising the externality.
政府可以通过征税（针对负外部性）或补贴（针对正外部性）来纠正外部性，这被称为外部性的内部化。
Published by TutorHao | KS3 Economics Revision Series | aleveler.com
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