📚 Year 8 OCR Economics: Quick Reference Formula & Theorem Handbook | Year 8 OCR 经济：公式定理速查手册
Welcome to your essential quick reference guide for Year 8 OCR Economics. This handbook gathers the key formulas, theorems and core concepts you need to build a solid foundation in economics. Whether you are revising for an end-of-topic test or simply keen to connect the dots between theory and real-world decisions, this resource breaks down each idea clearly with paired English and Chinese explanations.
欢迎使用这本 Year 8 OCR 经济必备速查手册。本手册汇集了你需要掌握的关键公式、定理和核心概念，助你打下坚实的经济学基础。无论你是在准备单元测试复习，还是想将理论与现实决策连接起来，本资源都会用清晰的中英双语解释逐一拆解每个概念。
1. Scarcity and Choice | 稀缺性与选择
Scarcity is the fundamental economic problem: unlimited wants but limited resources. Because we cannot have everything, choices must be made. This gives rise to the concept of opportunity cost.
稀缺性是基本的经济问题：无限的需求与有限的资源。因为我们不可能拥有一切，所以必须做出选择，这就产生了机会成本的概念。
The central theorem here is that every choice involves a trade-off – there is no such thing as a free lunch. Economics is the study of how individuals, firms and governments allocate these scarce resources to meet the most urgent needs and wants.
这里的核心定理是：每一个选择都伴随着权衡——天下没有免费的午餐。经济学正是研究个人、企业和政府如何分配这些稀缺资源以满足最迫切需求的学问。
2. Opportunity Cost | 机会成本
Opportunity cost is the value of the next best alternative that is given up when a decision is made. It is not the sum of all alternatives, only the single most valuable option forgone. Thinking at the margin helps decision-makers compare the additional benefit with the additional cost.
机会成本是指做出某个决策时所放弃的次优选择的价值。它不是所有替代选项的总和，仅仅是所放弃的最有价值的那一个选项。边际思维有助于决策者比较额外收益与额外成本。
Opportunity Cost = Value of Next Best Alternative Forgone
For example, if a government spends £10 million on a new road instead of a hospital, the opportunity cost is the health benefits lost from not building the hospital. A student choosing to play video games for an hour faces the opportunity cost of the studying or exercising they could have done instead.
例如，如果一个政府花费1000万英镑修建一条新路而不是一所医院，那么机会成本就是因未建医院而损失的医疗健康效益。一名学生选择玩一小时的电子游戏，其机会成本便是他本可以用于学习或锻炼的时间。
3. Factors of Production | 生产要素
The four factors of production are the inputs used to produce goods and services: land (natural resources), labour (human effort), capital (man-made aids such as machinery), and enterprise (the risk-taking ability that organises the other three). Each factor earns a reward: rent, wages, interest and profit respectively.
四种生产要素是用于生产商品和服务的投入：土地（自然资源）、劳动（人力）、资本（如机器等人造辅助品）以及企业（承担风险并组织其他三要素的能力）。每种要素获得相应的回报：地租、工资、利息和利润。
The theorem of factor mobility reminds us that some factors can move between uses more easily than others – for example, labour retraining takes time and capital equipment cannot usually be switched instantly. This influences how quickly an economy can adapt to change.
要素流动性定理提醒我们，某些要素在不同用途之间的转移比其他的更容易——例如，劳动力再培训需要时间，资本设备通常不能瞬间转换用途。这影响着经济体适应变化的速度。
4. Production Possibility Frontier (PPF) | 生产可能性边界
The PPF shows the maximum possible output combinations of two goods an economy can produce with its given resources and technology, when all resources are fully and efficiently employed. It is a powerful diagram to illustrate scarcity, choice and opportunity cost all at once.
生产可能性边界展示了一个经济体在资源和技术既定的情况下，当所有资源均得到充分有效使用时，能够生产的两种商品的最大产量组合。它是同时展示稀缺、选择和机会成本的强大图形工具。
Points on the curve represent productive efficiency; points inside represent inefficiency or unemployment; points outside are currently unattainable. An outward shift of the entire PPF can result from an increase in resources or technological progress.
曲线上的点代表生产效率；曲线内的点代表无效率或失业；曲线外的点暂时无法达到。整个 PPF 曲线向外移动可能源于资源增加或技术进步。
Slope of PPF = Opportunity Cost of Good X in terms of Good Y
The outward-bowed shape illustrates the law of increasing opportunity cost – as you produce more of one good, you must give up increasing amounts of the other because resources are not equally efficient in all activities. A straight-line PPF would indicate constant opportunity cost, which is rare in the real world.
向外凸出的形状解释了机会成本递增法则——当生产一种商品越多，你必须放弃的另一种商品数量会逐渐增加，因为资源并不能在所有活动中保持同等效率。一条直线的 PPF 则表明机会成本不变，这在现实中很少见。
5. Demand Law and Demand Curve | 需求法则与需求曲线
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. This inverse relationship is represented by a downward-sloping demand curve. The income effect and substitution effect support this law.
需求法则表明，在其他条件不变的情况下，商品价格上涨，需求量下降，反之亦然。这种反向关系表现为一条向下倾斜的需求曲线。收入效应和替代效应共同支持了这一法则。
A simple linear demand function can be written as:
Qd = a - bP
where Qd is quantity demanded, P is price, 'a' is the intercept (quantity demanded when price is zero), and 'b' is the slope coefficient showing the responsiveness of quantity demanded to price changes.
其中 Qd 为需求量，P 为价格，'a' 是截距（价格为0时的需求量），'b' 是斜率系数，反映需求量对价格变动的反应程度。
Shifts in the demand curve (rightward for increase, leftward for decrease) are caused by changes in non-price determinants such as income, tastes, population, and prices of related goods (substitutes and complements). A movement along the curve is caused only by a change in the good's own price.
需求曲线的移动（右移表示增加，左移表示减少）由非价格决定因素的变动引起，例如收入、偏好、人口以及相关商品（替代品和互补品）的价格。沿着曲线的移动则仅由商品自身价格的变化引起。
6. Supply Law and Supply Curve | 供给法则与供给曲线
The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied increases, and as the price falls, quantity supplied decreases. Producers are motivated by higher prices because these often mean greater potential profit. The supply curve slopes upward.
供给法则表明，在其他条件不变的情况下，商品价格上涨，供给量增加；价格下降，供给量减少。更高的价格激励生产者，因为这往往意味着更大的潜在利润。供给曲线向上倾斜。
A basic linear supply function:
Qs = c + dP
where Qs is quantity supplied, P is price, 'c' is the intercept (the quantity that would be supplied at a zero price, often negative or representing a minimum price threshold), and 'd' is the slope showing the change in quantity supplied per unit change in price.
其中 Qs 为供给量，P 为价格，'c' 为截距（价格为零时的供给量，通常为负值或代表启动供应的最低价格门槛），'d' 为斜率，表示价格每变动一单位时供给量的变动。
Shifts in supply result from changes in production costs, technology, taxes, subsidies, weather, and the number of sellers. A favourable condition, like a new cost-saving technology, shifts supply right; an input price increase shifts supply left.
供给曲线的移动源于生产成本、技术、税收、补贴、天气以及卖家数量的变化。有利的条件（如节约成本的新技术）使供给右移；投入品价格上涨则使供给左移。
7. Market Equilibrium | 市场均衡
Equilibrium occurs when quantity demanded equals quantity supplied. At this point, there is no tendency for the price to change because the plans of buyers and sellers match perfectly. Economists call this the market-clearing price.
均衡出现在需求量等于供给量时。此时价格没有变化的趋势，因为买卖双方的计划完全匹配。经济学家称之为市场出清价格。
Equilibrium Condition: Qd = Qs
Graphically, it is the intersection of demand and supply curves. If the market price is above equilibrium, a surplus occurs and firms cut prices to sell excess stocks. If the price is below equilibrium, a shortage arises and consumers bid the price up. These forces restore equilibrium automatically in free markets.
从图形看，均衡点是需求曲线和供给曲线的交点。如果市场价格高于均衡价格，会出现过剩，企业会降价以清空多余库存；如果低于均衡价格，出现短缺，消费者会抬高价格竞买。这些力量在自由市场中会自动恢复均衡。
8. Price Elasticity of Demand (PED) | 需求的价格弹性
PED measures how responsive quantity demanded is to a change in price. It helps firms predict changes in revenue and is vital for pricing decisions. The sign is always negative because of the law of demand, but we often use the absolute value.
需求的价格弹性衡量需求量对价格变动的反应程度。它帮助企业预测收入变化，对定价决策至关重要。由于需求法则，其符号总是负数，但我们通常使用绝对值进行讨论。
PED = % Change in Quantity Demanded ÷ % Change in Price
If |PED| > 1, demand is elastic – consumers are price-sensitive and a price cut increases total revenue. If |PED| < 1, demand is inelastic – a price rise boosts total revenue. If |PED| = 1, it is unit elastic, and revenue stays constant. Extreme cases: |PED| = 0 perfectly inelastic; |PED| = ∞ perfectly elastic.
若 |PED| > 1，需求富有弹性——消费者对价格敏感，降价会提高总收入。若 |PED| < 1，需求缺乏弹性——提价会增加总收入。若 |PED| = 1，为单位弹性，收入保持不变。极端情况：|PED| = 0 为完全无弹性；|PED| = ∞ 为完全弹性。
Year 8 students are often introduced to the midpoint (arc) method to avoid different elasticity values depending on the direction of change:
PED = (ΔQ / Q avg) ÷ (ΔP / P avg)
Here, Δ (delta) means 'change in', and Q avg and P avg are the averages of the initial and final quantities and prices. This formula gives a consistent measure along a demand segment.
其中 Δ（德尔塔）意为“变化量”，Q avg 与 P avg 分别是初始和最终数量、价格的平均值。该公式沿需求曲线一段得出一致的弹性度量。
9. Costs, Revenue and Profit | 成本、收益与利润
Businesses need to understand cost structures to survive and grow. Total cost (TC) is the sum of fixed costs (FC) and variable costs (VC). Fixed costs do not change with output in the short run; variable costs do.
企业需要理解成本结构才能生存与发展。总成本（TC）是固定成本（FC）和可变成本（VC）的总和。短期中固定成本不随产量变化，可变成本则随产量改变。
TC = FC + VC
Average cost (AC) tells us the cost per unit produced, which is vital for setting prices:
AC = TC ÷ Q
Total revenue (TR) is the money received from sales:
TR = Price × Quantity = P × Q
Profit is the ultimate goal for most firms and
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