📚 KS3 CCEA Economics: Quick Reference Handbook of Formulas and Theorems | KS3 CCEA 经济：公式定理速查手册
This handbook brings together the essential concepts, economic laws, and formulas you need to master for the CCEA Key Stage 3 Economics syllabus. Each section is presented in a clear, paired English-Chinese format, giving you quick access to definitions, theorems, and calculations ranging from scarcity and opportunity cost to supply and demand, costs, revenue, budgeting, and interest. Use it as a rapid revision tool to strengthen your understanding and recall of the key building blocks of economics.
本手册汇集了 CCEA KS3 经济课程需要掌握的核心概念、经济定律和公式。每一个部分都以清晰的英中对照形式呈现，为你提供从稀缺性和机会成本到供需关系、成本、收益、预算和利息等定义、定理与计算的快速查阅。你可以将其用作快速复习工具，巩固对经济学关键基石的理解和记忆。
1. Scarcity and the Basic Economic Problem | 稀缺性与基本经济问题
The fundamental economic problem is that our resources are limited, yet human wants are endless. This condition of scarcity forces individuals, firms, and governments to make choices. The central theorem: every society must answer three basic questions – what to produce, how to produce, and for whom to produce.
基本经济问题在于资源有限而人的欲望无止境。这种稀缺性迫使个人、企业和政府作出选择。核心定理：每个社会都必须回答三个基本问题——生产什么、如何生产以及为谁生产。
- Scarcity means there are not enough resources to satisfy all wants. / 稀缺性意味着没有足够的资源来满足所有欲望。
- Choice is necessary because of scarcity. / 由于稀缺性，选择不可避免。
- The three economic agents (households, firms, government) all face this problem. / 三类经济主体（家庭、企业、政府）都面临这个问题。
2. Opportunity Cost | 机会成本
Opportunity cost is the value of the next best alternative that must be given up when making a choice. The theorem states that the real cost of any decision is not just the money spent, but the benefit lost from the best foregone option. For instance, if a student decides to spend an hour studying economics instead of playing sport, the opportunity cost is the enjoyment and fitness benefit of that sport.
机会成本是做出选择时必须放弃的次优替代方案的价值。定理指出，任何决策的真实成本不仅是花掉的钱，更是所放弃的最佳选项带来的利益损失。例如，一名学生决定用一小时学习经济而不是进行体育运动，那么机会成本就是那项运动带来的乐趣和健康收益。
Opportunity Cost = Value of the Next Best Alternative Foregone
机会成本 = 放弃的次优替代方案的价值
3. The Law of Demand | 需求定理
The law of demand states that, ceteris paribus (all other factors held constant), as the price of a good rises, the quantity demanded falls, and as the price falls, the quantity demanded rises. This inverse relationship is one of the most important principles in economics, producing a downward-sloping demand curve. The income effect and substitution effect help explain why consumers buy less at higher prices.
需求定理指出，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。这种反向关系是经济学中最重要的原理之一，形成一条向下倾斜的需求曲线。收入效应和替代效应有助于解释为何消费者在价格更高时购买更少。
- Demand curve slopes downward from left to right. / 需求曲线从左向右下方倾斜。
- Only a change in price causes a movement along the demand curve (change in quantity demanded). / 只有价格变动才会引起沿需求曲线的移动（需求量变动）。
- A change in any other factor (e.g. income, tastes) shifts the entire demand curve. / 任何其他因素（如收入、偏好）的变动会平移整条需求曲线。
4. The Law of Supply | 供给定理
The law of supply says that, ceteris paribus, as the price of a good increases, the quantity supplied also increases. Producers are motivated by higher potential profits to offer more for sale. This results in an upward-sloping supply curve. The relationship reflects the profit incentive and the fact that producing more often involves higher costs, which firms will only incur if the price is high enough to cover them.
供给定理指出，在其他条件不变时，商品价格上升，供给量也随之增加。生产者受到更高潜在利润的驱动，愿意出售更多产品，从而形成向上倾斜的供给曲线。这种关系体现了利润激励，也反映出要生产更多往往需承担更高成本，只有价格足够高时企业才愿承担这些成本。
- Supply curve slopes upward from left to right. / 供给曲线从左向右上方倾斜。
- A change in the good's own price causes a movement along the supply curve. / 商品自身价格的变动引起沿供给曲线的移动。
- Shifts of the supply curve happen when production costs, technology, taxes, or the number of sellers change. / 当生产成本、技术、税收或卖者数量变化时，供给曲线发生平移。
5. Market Equilibrium | 市场均衡
Market equilibrium is the point where the demand and supply curves intersect. At this point, the quantity that consumers are willing and able to buy exactly equals the quantity that producers are willing and able to sell. There is no tendency for price to change unless an external factor shifts either curve. The equilibrium condition and its price-quantity combination eliminate shortages and surpluses.
市场均衡是需求曲线与供给曲线的交点。在这一点，消费者愿意且能够购买的数量恰好等于生产者愿意且能够出售的数量。除非外部因素使得任一曲线平移，否则价格不会改变。均衡条件及其对应的价格-数量组合消除了短缺和过剩。
Equilibrium Condition: Qd = Qs
均衡条件：需求量 = 供给量
If price is above equilibrium, a surplus (excess supply) pushes price down. If price is below equilibrium, a shortage (excess demand) pushes price up. / 如果价格高于均衡价格，则出现过剩（超额供给），促使价格下降。如果价格低于均衡价格，则出现短缺（超额需求），促使价格上升。
6. 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 the good’s own price. The formula uses percentage changes to allow comparisons across different goods and units.
需求价格弹性衡量需求量对商品自身价格变化的反应程度。该公式采用百分比变化，以便在不同商品和单位之间进行比较。
PED = (% Change in Quantity Demanded) ÷ (% Change in Price)
PED = (需求量变动百分比) ÷ (价格变动百分比)
Key elasticity values:
- PED > 1: elastic demand – quantity demanded changes more than proportionately to price change. / 大于1：富有弹性——需求量变化比例大于价格变化比例。
- PED < 1: inelastic demand – quantity demanded changes less than proportionately. / 小于1：缺乏弹性——需求量变化比例小于价格变化比例。
- PED = 1: unit elastic demand – quantity demanded changes exactly proportionately. / 等于1：单位弹性——需求量与价格同比例变化。
- PED = 0: perfectly inelastic – quantity demanded does not change with price. / 等于0：完全无弹性——价格变化时需求量不变。
- PED = ∞: perfectly elastic – even a tiny price change causes quantity demanded to fall to zero. / 无穷大：完全弹性——极小的价格变化就会使需求量降为零。
7. Types of Costs | 成本分类
Firms face different categories of costs in the short run. Fixed costs do not vary with the level of output (e.g. rent, permanent staff salaries). Variable costs change directly with output (e.g. raw materials, energy). The total cost is simply the sum of the two.
企业在短期内面临不同类型的成本。固定成本不随产量水平变动（例如租金、固定员工的工资）。可变成本直接随产量变动（例如原材料、能源）。总成本就是二者之和。
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)
Average total cost is the cost per unit of output and is useful for pricing decisions.
平均总成本是每单位产量的成本，对定价决策很有用。
Average Total Cost (ATC) = TC ÷ Quantity (Q)
平均总成本 (ATC) = TC ÷ 产量 (Q)
- As output rises, total fixed cost stays the same, so average fixed cost falls. / 随着产量增加，总固定成本不变，因此平均固定成本下降。
- Total variable cost rises with output, so average variable cost may eventually rise due to diminishing returns. / 总可变成本随产量上升而上升，因此由于收益递减，平均可变成本最终可能上升。
8. Revenue and Profit | 收益与利润
Total revenue is the income a firm receives from selling its product. It is calculated by multiplying the selling price per unit by the quantity sold. Profit is the difference between total revenue and total cost and is the primary objective of most firms in a market economy.
总收益是企业销售产品所获得的收入，由单位售价乘以销量计算得出。利润是总收益与总成本之差，也是市场经济中大多数企业的主要目标。
Total Revenue (TR) = Price (P) × Quantity Sold (Q)
总收益 (TR) = 价格 (P) × 销量 (Q)
Profit = Total Revenue (TR) – Total Cost (TC)
利润 = 总收益 (TR) − 总成本 (TC)
the firm makes a profit. If TR < TC, it makes a loss. If TR = TC, the firm breaks even. / 若 TR > TC，企业盈利；若 TR < TC，企业亏损；若 TR = TC，企业盈亏平衡。
9. Budgeting | 预算
A budget is a financial plan showing expected income and planned expenditure over a period of time. It applies to individuals, households, firms, and governments. The basic budget equation shows that all income must be either spent or saved.
预算是一段时期内展示预期收入和计划支出的财务计划，适用于个人、家庭、企业和政府。基本的预算等式表明全部收入要么用于支出，要么用于储蓄。
Income = Expenditure + Savings
收入 = 支出 + 储蓄
- Budget surplus: Income > Expenditure. / 预算盈余：收入 > 支出。
- Budget deficit: Expenditure > Income. / 预算赤字：支出 > 收入。
- Balanced budget: Income = Expenditure. / 预算平衡：收入 = 支出。
Good budgeting helps to avoid debt and to achieve financial goals. / 良好的预算有助于避免债务并实现财务目标。
10. Savings and Interest | 储蓄与利息
Savings represent the part of disposable income that is not spent on consumption. When savings are deposited in a bank, they can earn interest – a payment for the use of money over time. The simple interest formula is widely used at KS3 level to calculate interest earned or the cost of borrowing.
储蓄代表可支配收入中未用于消费的部分。将储蓄存入银行可获得利息——这是因资金随时间使用而支付的报酬。单利公式在 KS3 阶段被广泛用来计算利息收入或借贷成本。
Simple Interest (I) = Principal (P) × Rate (r) × Time (t)
单利 (I) = 本金 (P) × 利率 (r) × 时间 (t)
Example: If you save £200 at an annual interest rate of 3% for 4 years, the simple interest earned is I = 200 × 0.03 × 4 = £24. / 例如：若以3%的年利率储蓄200英镑，期限4年，单利收入为 I = 200 × 0.03 × 4 = 24英镑。
11. Production Possibility Frontier (PPF) | 生产可能性边界
The production possibility frontier is a model that shows the maximum possible output combinations of two goods or services that an economy can produce using all available resources and the best technology. The PPF illustrates the concepts of scarcity, choice, and opportunity cost very clearly.
生产可能性边界是一种模型，展示一个经济体利用全部可用资源和最佳技术所能生产的两种商品或服务的最大产出组合。PPF 非常清晰地说明了稀缺性、选择和机会成本这三个概念。
- Points on the PPF curve represent efficient production. / PPF曲线上的点表示有效率的生产。
- Points inside the curve show inefficient use of resources (unemployment or underemployment). / 曲线内的点表示资源利用无效率（失业或未充分就业）。
- Points outside the curve are unattainable with current resources. / 曲线外的点在当前资源下无法实现。
- Moving along the curve involves an opportunity cost – to get more of one good, some of the other must be sacrificed. / 沿曲线移动涉及机会成本——要多得到一种商品，就必须牺牲部分另一种商品。
- Economic growth shifts the entire PPF outward. / 经济增长将整条 PPF 向外推移。
12. Shifts in Demand and Supply | 需求与供给的移动
Beyond price, many other factors can cause the entire demand or supply curve to shift. When a demand curve shifts, both equilibrium price and quantity change in the same direction as the shift. When a supply curve shifts, the equilibrium price moves in the opposite direction while quantity moves in the same direction as the shift.
除了价格，许多其他因素也会导致整条需求或供给曲线平移。当需求曲线平移时，均衡价格和数量都与移动方向同向变化。当供给曲线平移时，均衡价格与移动方向反向变化，而数量则同向变化。
Factors shifting demand (mnemonic: PIRATES):
- Population (number of buyers) / 人口（买者数量）
- Income (normal vs inferior goods) / 收入（正常品与低档品）
- Related goods' prices (substitutes and complements) / 相关商品的价格（替代品与互补品）
- Advertising and tastes / 广告与偏好
- Tastes and fashion / 品味与时尚
- Expectations of future prices / 对未来价格的预期
- Seasons and weather / 季节与天气
Factors shifting supply: changes in costs of production, improvements in technology, taxes and subsidies, number of suppliers, and weather conditions for agricultural goods. / 导致供给移动的因素：生产成本的变化、技术进步、税收和补贴、供应商数量以及影响农产品的天气状况。
