📚 KS3 Cambridge Economics: Quick-Reference Formulas & Theorems Handbook | KS3 剑桥经济：公式定理速查手册
Mastering the essential formulas and economic principles at the KS3 level builds a solid foundation for Cambridge IGCSE and beyond. This quick-reference handbook summarises the key theorems, definitions, and calculation methods you need to tackle basic economic problems, understand how markets work, and interpret data confidently. Each section pairs a concise English explanation with its Chinese counterpart, making it easier to switch between languages and reinforce your learning.
掌握 KS3 阶段的核心经济学公式和基本原理，能为剑桥 IGCSE 及后续学习打下坚实基础。这本速查手册总结了关键定理、定义和计算方法，帮助你解决基本经济问题、理解市场运作机制并自信地解读数据。每个小节都配有简明的中英文对照讲解，方便你在双语环境中巩固知识。
1. Scarcity and the Basic Economic Problem | 稀缺性与基本经济问题
Scarcity lies at the heart of economics: resources are limited, but human wants are virtually unlimited. Because we cannot have everything we desire, individuals, businesses, and governments must make choices about how to use scarce resources. This leads to the three fundamental economic questions: What goods and services should be produced? How should they be produced? For whom should they be produced? Scarcity creates the need for trade-offs and the concept of opportunity cost.
稀缺性是经济学的核心问题：资源是有限的，而人类的欲望几乎是无限的。正因为我们无法拥有一切，个人、企业和政府必须就如何使用稀缺资源做出选择。这就引出了三个基本经济问题：应该生产什么商品和服务？如何生产？为谁生产？稀缺性导致了权衡取舍的必要性以及机会成本的概念。
The basic economic problem can be summarised by the fact that resources (land, labour, capital, and enterprise) are scarce, yet demands for goods and services are infinite. Economics studies how these scarce resources are allocated efficiently to satisfy as many wants as possible.
基本经济问题可以概括为：资源（土地、劳动、资本和企业才能）是稀缺的，而对商品和服务的需求是无限的。经济学研究如何有效配置这些稀缺资源，以尽可能满足最多的欲望。
2. Opportunity Cost | 机会成本
Opportunity cost is the value of the next best alternative that is sacrificed when a choice is made. Every decision involving scarce resources carries an opportunity cost, whether it is a student choosing to study instead of working part-time, or a government deciding to build a road instead of a hospital. It is not simply the monetary cost, but the most desirable forgone option.
机会成本是指做出选择时所放弃的次优选项的价值。每一个涉及稀缺资源的决定都伴随着机会成本，无论是学生选择学习而不是兼职工作，还是政府决定修路而不是建医院。机会成本不仅仅是货币成本，而是被放弃的最理想选项。
Opportunity Cost = Value of the Next Best Alternative Forgone
机会成本 = 放弃的次优选择的价值
Understanding opportunity cost helps rational decision-makers compare the true cost of different actions. For example, if a farmer can grow either wheat or barley on a plot of land, choosing wheat means the opportunity cost is the profit that could have been earned from barley.
理解机会成本有助于理性决策者比较不同行动的真实代价。例如，如果一个农民可以在同一块土地上种小麦或大麦，选择种小麦的机会成本就是原本种大麦能获得的利润。
3. Factors of Production | 生产要素
Resources used to produce goods and services are classified into four factors of production: land, labour, capital, and enterprise. Each factor earns a corresponding reward.
用于生产商品和服务的资源被分为四种生产要素：土地、劳动、资本和企业才能。每种要素都获得相应的报酬。
- Land – natural resources (reward: rent)
- Labour – human effort (reward: wages)
- Capital – man-made tools and machinery (reward: interest)
- Enterprise – the risk-taking ability to combine the other three factors (reward: profit)
中文解释如下：
- 土地——自然资源（报酬：租金）
- 劳动——人力投入（报酬：工资）
- 资本——人造的工具和机器（报酬：利息）
- 企业才能——承担风险、组合其他三种要素的能力（报酬：利润）
These factors are scarce and have alternative uses. An efficient economy uses them in such a way that maximum output is achieved, which directly relates to the production possibility frontier (PPF) and the concepts of efficiency and growth.
这些要素是稀缺的，且具有多种用途。一个高效的经济体会以能实现最大产出的方式使用它们，这直接关系到生产可能性边界（PPF）以及效率和增长的概念。
4. Demand and the Law of Demand | 需求与需求定律
Demand refers to the willingness and ability of consumers to purchase a good or service at various prices over a given period. The law of demand states that, ceteris paribus, as the price of a good rises, the quantity demanded falls, and vice versa. This inverse relationship gives the demand curve a downward slope.
需求是指消费者在不同价格水平下愿意并能够购买某种商品或服务的能力和意愿，且限定在一定时期内。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。这种反比关系使需求曲线呈向下倾斜的趋势。
Factors that can shift the entire demand curve include changes in income, tastes and preferences, prices of related goods (substitutes and complements), expectations, and the number of buyers. A rightward shift represents an increase in demand; a leftward shift indicates a decrease.
能够使整条需求曲线移动的因素包括收入的变化、偏好与口味、相关商品（替代品和互补品）的价格、预期以及买家人数。需求曲线向右移动代表需求增加，向左移动代表需求减少。
5. 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 different prices over a specific period. The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied rises. This positive relationship gives the supply curve an upward slope.
供给是指生产者在不同价格水平下愿意并能够出售的商品或服务的数量，且限定在一定时期内。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，供给量上升。这种正比关系使供给曲线呈向上倾斜的趋势。
Shifts in the supply curve are caused by changes in production costs, technology, taxes and subsidies, prices of other goods, expectations, and the number of sellers. An increase in supply shifts the curve to the right; a decrease shifts it to the left.
供给曲线的移动由生产成本、技术、税收与补贴、其他商品的价格、预期以及卖家人数等因素引起。供给增加会使曲线向右移动，供给减少则向左移动。
6. Market Equilibrium | 市场均衡
Market equilibrium occurs at the price where the quantity demanded equals the quantity supplied. At this equilibrium price (P∗), there is no surplus or shortage, and the market clears. Any imbalance between demand and supply exerts pressure on price to adjust back towards equilibrium.
市场均衡出现在需求量等于供给量的价格水平上。在这个均衡价格（P∗）下，既没有过剩也没有短缺，市场出清。任何供需失衡都会对价格产生压力，使其向均衡方向调整。
At equilibrium: Qd = Qs
均衡条件：需求量（Qd）= 供给量（Qs）
If the price is set above equilibrium, a surplus arises and producers lower prices. If the price is below equilibrium, a shortage occurs and consumers bid prices up. Understanding equilibrium helps predict how markets respond to changes in determinants of demand and supply.
如果价格设定在均衡水平之上，就会出现过剩，生产者会降低价格；如果价格低于均衡水平，就会出现短缺，消费者会抬高价格。理解均衡有助于预测市场如何对需求和供给的决定因素变化做出反应。
7. 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. It is calculated as the percentage change in quantity demanded divided by the percentage change in price. The coefficient of PED indicates whether demand is elastic, inelastic, or unit elastic.
需求价格弹性衡量需求量对价格变化的反应程度。它的计算方法是需求量变化的百分比除以价格变化的百分比。PED 系数的大小表明需求是富有弹性、缺乏弹性还是单位弹性。
PED = %ΔQd / %ΔP
PED = (ΔQd ÷ Qd) ÷ (ΔP ÷ P)
If PED > 1, demand is elastic (luxury goods); if PED < 1, demand is inelastic (necessities); if PED = 1, demand is unit elastic. Determinants of PED include availability of substitutes, proportion of income spent, and time period. The concept is crucial for firms when setting prices to maximise total revenue.
如果 PED > 1，需求富有弹性（奢侈品）；如果 PED < 1，需求缺乏弹性（必需品）；如果 PED = 1，需求具有单位弹性。影响 PED 的因素包括替代品的可获得性、支出占收入的比例以及时间长短。这一概念对企业制定价格以实现总收益最大化至关重要。
8. 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. It is defined as the percentage change in quantity supplied divided by the percentage change in price. PES helps determine how easily producers can adjust output when market conditions change.
供给价格弹性衡量供给量对价格变化的反应程度。它的定义是供给量变化的百分比除以价格变化的百分比。PES 有助于判断生产者在市场状况变化时调整产量的难易程度。
PES = %ΔQs / %ΔP
PES = (ΔQs ÷ Qs) ÷ (ΔP ÷ P)
A PES greater than 1 indicates elastic supply (producers can increase output quickly); less than 1 indicates inelastic supply (limited ability to adjust). Factors affecting PES include time period, spare production capacity, availability of raw materials, and ease of storing inventory.
PES 大于 1 表明供给富有弹性（生产者能迅速增加产量）；小于 1 表明供给缺乏弹性（调整能力有限）。影响 PES 的因素包括时间长短、闲置产能、原材料的可获得性以及储存便利性。
9. Total Revenue | 总收益
Total revenue is the total income a firm receives from selling its goods or services. It is calculated by multiplying the price per unit by the quantity sold. Understanding the relationship between total revenue and price elasticity helps businesses make profitable pricing decisions.
总收益是企业通过销售商品或服务获得的总收入。它等于单位价格乘以销售量。理解总收益与价格弹性之间的关系有助于企业做出盈利的定价决策。
Total Revenue (TR) = Price (P) × Quantity (Q)
总收益（TR）= 价格（P）× 数量（Q）
If demand is elastic, a price cut raises total revenue because the percentage increase in quantity demanded outweighs the percentage drop in price. If demand is inelastic, a price rise increases total revenue. Firms must therefore consider PED when estimating the effect of price changes on revenue.
如果需求富有弹性，降价会提高总收益，因为需求量增加的百分比超过了价格下降的百分比。如果需求缺乏弹性，提价会增加总收益。因此，企业在预估价格变动对收益的影响时，必须考虑 PED。
10. Costs of Production | 生产成本
Production costs are divided into fixed costs (FC), which do not change with output, and variable costs (VC), which vary directly with the level of production. Total cost (TC) is the sum of both. Analysing average costs helps firms determine the efficiency of their production processes.
生产成本分为固定成本（FC，不随产量变化）和变动成本（VC，随产量直接变化）。总成本（TC）是两者之和。分析平均成本有助于企业判断生产过程的效率。
The key cost formulas are summarised below:
关键的成本公式总结如下：
| Formula (English) | 公式 (中文) |
| Total Cost (TC) = FC + VC | 总成本 (TC) = 固定成本 + 变动成本 |
| Average Total Cost (ATC) = TC ÷ Q | 平均总成本 (ATC) = 总成本 ÷ 产量 |
| Average Fixed Cost (AFC) = FC ÷ Q | 平均固定成本 (AFC) = 固定成本 ÷ 产量 |
| Average Variable Cost (AVC) = VC ÷ Q | 平均变动成本 (AVC) = 变动成本 ÷ 产量 |
As output expands, AFC falls continuously because fixed cost is spread over more units. AVC and ATC typically fall at first due to increasing returns, then rise due to diminishing returns. Firms aim to produce at an output level where costs per unit are minimised.
随着产量扩大，AFC 持续下降，因为固定成本被分摊到更多单位上。AVC 和 ATC 通常因报酬递增先下降，之后因报酬递减而上升。企业力求在单位成本最低的产量水平上生产。
11. Profit | 利润
Profit is the financial gain a business makes, calculated as the difference between total revenue and total cost. It is the primary incentive for entrepreneurs and a signal for resource allocation. There are two common measures: accounting profit (explicit costs only) and economic profit (including opportunity costs).
利润是企业的财务收益，等于总收益减去总成本。它是企业家的主要动力，也是资源配置的信号。常见的衡量方式有两种：会计利润（仅考虑显性成本）和经济利润（包括机会成本）。
Profit = Total Revenue (TR) – Total Cost (TC)
利润 = 总收益 (TR) – 总成本 (TC)
If TR > TC, the firm earns a positive profit (supernormal or normal profit, depending on whether opportunity costs are covered). If TR < TC, the firm makes a loss. Understanding profit maximisation is essential when applying marginal analysis: firms maximise profit where marginal cost equals marginal revenue (MC = MR).
若 TR > TC，企业获得正利润（正常利润或超额利润，取决于是否覆盖了机会成本）；若 TR < TC，企业发生亏损。理解利润最大化对于应用边际分析至关重要：企业在边际成本等于边际收益（MC = MR）时实现利润最大化。
12. Economic Growth and GDP | 经济增长与 GDP
Economic growth refers to an increase in the output of goods and services in an economy over time, typically measured by the percentage change in real Gross Domestic Product (GDP). GDP represents the total market value of all final goods and services produced within a country in a given period. A growing GDP generally indicates rising living standards.
经济增长是指一个经济体系在一定时期内商品和服务产出的增加，通常以实际国内生产总值（GDP）的百分比变化来衡量。GDP 代表一个国家在一定时期内生产的所有最终商品和服务的市场总价值。GDP 增长通常意味着生活水平的提高。
GDP = C + I + G + (X – M)
支出法 GDP = 消费 (C) + 投资 (I) + 政府支出 (G) + (出口 X – 进口 M)
Where C is private consumption, I is investment, G is government spending, X is exports, and M is imports. Net exports (X – M) add to GDP when a country exports more than it imports. Sustainable growth requires productivity improvements, investment in capital, and technological progress.
其中 C 为私人消费，I 为投资，G 为政府支出，X 为出口，M 为进口。当一个国家出口大于进口时，净出口（X – M）会增加 GDP。可持续增长需要提高生产率、进行资本投资和技术进步。
Policymakers use GDP data alongside other indicators like unemployment and inflation to design appropriate fiscal and monetary policies. For KS3 students, grasping this expenditure formula connects microeconomic decisions to the broader macroeconomy.
政策制定者利用 GDP 数据以及失业和通货膨胀等其他指标来制定合适的财政政策与货币政策。对于 KS3 学生而言，掌握这一支出法公式能将微观经济决策与更广泛的宏观经济联系起来。
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