📚 Year 9 WJEC Economics: Formula & Theorem Quick Reference Guide | 九年级WJEC经济：公式定理速查手册
Welcome to your Year 9 WJEC Economics formula and theorem quick reference guide. This handbook pulls together the essential definitions, relationships and equations you will meet as you explore how people, businesses and governments make choices. Use it to check formulae, reinforce your understanding and build confidence for class tests and end-of-year assessments.
欢迎使用九年级WJEC经济学公式定理速查手册。本手册汇集了你在探索个人、企业和政府如何做出选择时会遇到的核心定义、关系式和方程。用它来核对公式、巩固理解，并为课堂测验和年终考试增强信心。
1. The Basic Economic Problem & Scarcity | 基本经济问题与稀缺性
The basic economic problem arises because resources (land, labour, capital and enterprise) are finite, while human wants are unlimited. Scarcity forces every economy to answer three key questions: what to produce, how to produce and for whom to produce.
基本经济问题源于资源（土地、劳动、资本和企业家才能）有限而人的欲望无限。稀缺性迫使每个经济体回答三个关键问题：生产什么、如何生产以及为谁生产。
Every choice involves an opportunity cost — the value of the next best alternative forgone. Even free goods given away at zero price have an opportunity cost for the provider.
每个选择都涉及机会成本——即被放弃的次优替代品的价值。即便是以零价格赠送的免费商品，对供应方而言也存在机会成本。
2. Demand & 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 different prices over a given period, ceteris paribus (other things being equal).
需求是指在给定时期内、其他条件不变的情况下，消费者愿意并且能够以不同价格购买的商品或服务的数量。
The law of demand states that, ceteris paribus, as the price of a good rises, the quantity demanded falls; as price falls, quantity demanded rises. This inverse relationship gives the demand curve its downward slope.
需求定律指出，在其他条件不变时，商品价格上升，需求量下降；价格下降，需求量上升。这种反向关系使得需求曲线向下倾斜。
Factors that shift the whole demand curve (changes in demand) include income, tastes, advertising, prices of substitutes and complements, population size and expectations. A change in price causes a movement along the curve (change in quantity demanded).
导致整条需求曲线移动（需求变动）的因素包括收入、偏好、广告、替代品和互补品的价格、人口规模和预期。价格变化则引起沿需求曲线的移动（需求量变动）。
Demand function (simplified): Qd = f(P, I, Ps, Pc, T, ...)
需求函数（简化）：Qd = f(P, I, Ps, Pc, T, ...)
3. Supply & 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 given period, ceteris paribus.
供给是指在给定时期内、其他条件不变的情况下，生产者愿意并且能够以不同价格提供出售的商品或服务的数量。
The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied rises; as price falls, quantity supplied falls. This positive relationship creates an upward-sloping supply curve.
供给定律指出，在其他条件不变时，商品价格上升，供给量上升；价格下降，供给量下降。这种正相关关系形成向上倾斜的供给曲线。
Supply shifts are caused by changes in production costs, technology, taxes, subsidies, number of sellers, weather (for agricultural products) and expectations. A price change moves the quantity supplied along the existing curve.
供给曲线的移动由生产成本、技术、税收、补贴、卖者数量、天气（农产品）和预期等因素变化引起。价格变化则使供给量沿现有曲线移动。
Supply function (simplified): Qs = f(P, C, Tech, Tax, Sub, ...)
供给函数（简化）：Qs = f(P, C, Tech, Tax, Sub, ...)
4. Market Equilibrium & The Price Mechanism | 市场均衡与价格机制
Market equilibrium occurs where quantity demanded equals quantity supplied at a certain price. There is no tendency for change unless an external factor shifts either curve.
市场均衡出现在需求量等于供给量的某一价格水平上。除非外部因素使曲线移动，否则不存在变化的趋势。
Equilibrium condition: Qd = Qs
均衡条件：Qd = Qs
When the market price is above equilibrium, a surplus (excess supply) exists, pushing price down. When the price is below equilibrium, a shortage (excess demand) drives price up. This automatic adjustment is the price mechanism in action.
当市场价格高于均衡时，出现过剩（超额供给），迫使价格下降。当价格低于均衡时，出现短缺（超额需求），推动价格上升。这种自动调节就是价格机制的作用。
5. Price Elasticity of Demand (PED) | 需求的价格弹性
Price elasticity of demand measures how responsive quantity demanded is to a change in price. It helps firms predict the effect of price changes on total revenue.
需求的价格弹性衡量需求量对价格变化的反应程度。它帮助企业预测价格变化对总收入的影响。
PED = %ΔQd ÷ %ΔP
需求的价格弹性 = 需求量的百分比变化 ÷ 价格的百分比变化
In formula notation, %ΔQd = (ΔQd / initial Qd) × 100 and % / initial P) × 100. The result is usually negative, but economists often ignore the sign.
用符号表示，%ΔQd = (ΔQd / 初始Qd) × 100，%ΔP = (ΔP / 初始P) × 100。结果通常为负值，但经济学家通常忽略符号取其绝对值。
PED > 1: elastic demand — quantity demanded changes by a larger percentage than price. Lowering price raises total revenue.
PED > 1：弹性需求 — 需求量变化的百分比大于价格变化的百分比。降价会提高总收入。
PED < 1: inelastic demand — quantity demanded changes by a smaller percentage than price. Lowering price reduces total revenue.
PED < 1：缺乏弹性需求 — 需求量变化的百分比小于价格变化的百分比。降价会降低总收入。
PED = 1: unitary elastic — price and quantity demanded perfectly offset each other, leaving total revenue unchanged.
PED = 1：单位弹性 — 价格变化与需求量变化正好抵消，总收入不变。
6. Income Elasticity of Demand (YED) | 需求的收入弹性
Income elasticity of demand measures how quantity demanded responds to a change in consumers' income. It tells us whether a good is a normal good or an inferior good.
需求的收入弹性衡量需求量对消费者收入变化的反应。它告诉我们一种商品是正常品还是劣等品。
YED = %ΔQd ÷ %ΔY
需求的收入弹性 = 需求量的百分比变化 ÷ 收入的百分比变化
YED > 0: normal good — demand rises when income rises. Luxury goods tend to have YED > 1.
YED > 0：正常品 — 收入增加时需求上升。奢侈品的YED通常大于1。
YED < 0: inferior good — demand falls when income rises (e.g. own-brand basics).
YED < 0：劣等品 — 收入增加时需求下降（如自有品牌的基础商品）。
YED near 0: income-inelastic — demand barely changes with income (e.g. salt).
YED接近0：收入非弹性 — 需求几乎不随收入变化（如食盐）。
7. Price Elasticity of Supply (PES) | 供给的价格弹性
Price elasticity of supply measures how responsive quantity supplied is to a change in price. It largely depends on how quickly firms can increase output.
供给的价格弹性衡量供给量对价格变化的反应程度。它主要取决于企业增加产出的速度。
PES = %ΔQs ÷ %ΔP
供给的价格弹性 = 供给量的百分比变化 ÷ 价格的百分比变化
If PES > 1, supply is elastic — producers can increase output quickly without a large rise in costs. If PES < 1, supply is inelastic — it is hard to boost production in the short run (e.g. crops or concert tickets).
如果PES > 1，供给富有弹性—生产者能快速增加产出而成本不会大幅上升。如果PES < 1，供给缺乏弹性—短期内很难提高产量（如农作物或音乐会门票）。
8. Costs of Production | 生产成本
To understand profit, firms must categorise and track their costs. In the short run, some costs are fixed while others vary with output.
要理解利润，企业必须对成本进行分类和追踪。在短期内，部分成本固定，其他成本随产出变动。
| Cost term | Formula | Cost term (中文) |
|---|---|---|
| Total Cost (TC) | TFC + TVC | 总成本 |
| Total Fixed Cost (TFC) | Sum of costs that do not vary with output | 总固定成本 |
| Total Variable Cost (TVC) | Sum of costs that change with output | 总可变成本 |
| Average Total Cost (ATC) | TC ÷ Q | 平均总成本 |
| Average Fixed Cost (AFC) | TFC ÷ Q | 平均固定成本 |
| Average Variable Cost (AVC) | TVC ÷ Q | 平均可变成本 |
| Marginal Cost (MC) | ΔTC ÷ ΔQ | 边际成本 |
The marginal cost curve usually falls briefly then rises as output expands, reflecting the law of diminishing returns.
边际成本曲线通常先短暂下降，然后随着产量扩大而上升，反映了边际收益递减规律。
9. Revenue & Profit | 收入与利润
Revenue is the income a firm receives from selling its output. Profit is what remains after covering all costs.
收入是企业销售产品所获得的款项。利润是扣除所有成本后的剩余。
Total Revenue (TR) = Price (P) × Quantity sold (Q)
总收入 (TR) = 价格 (P) × 销售量 (Q)
Average Revenue (AR) = TR ÷ Q = P
平均收入 (AR) = TR ÷ Q = P
Profit (π) = TR − TC
利润 (π) = 总收入 − 总成本
zero economic profit); supernormal profit occurs when TR > TC. Losses happen when TC > TR.
正常利润指 TR = TC（经济利润为零）；超额利润指 TR > TC。当 TC > TR 时企业出现亏损。
10. Market Structures Overview | 市场结构概述
Market structures describe the competitive environment in which firms operate. The four main types are perfect competition, monopolistic competition, oligopoly and monopoly.
市场结构描述了企业所处的竞争环境。四种主要类型是完全竞争、垄断竞争、寡头垄断和完全垄断。
many small firms, identical products, no barriers to entry, price takers. AR = MR = Price.
完全竞争：众多小企业，同质产品，无进入壁垒，价格接受者。AR = MR = 价格。
Monopolistic competition: many firms, slightly differentiated products, low barriers, some price-setting power.
垄断竞争：众多企业，产品略有差异，低进入壁垒，有一定定价能力。
Oligopoly: a few dominant firms, high barriers, interdependent decision-making.
寡头垄断：少数主导企业，高进入壁垒，决策相互依赖。
Monopoly: a single seller, unique product, very high barriers, significant price-setting power.
完全垄断：单一卖家，独一无二的产品，极高的进入壁垒，强大的定价能力。
In reality, most markets sit between perfectly competitive and monopoly extremes. Knowing the structure helps predict pricing and output behaviour.
现实中，大多数市场介于完全竞争与垄断两个极端之间。了解市场结构有助于预测定价和产量行为。
11. Government Intervention | 政府干预
Governments intervene in markets to correct market failures, redistribute income or achieve wider social objectives. Common tools include taxation, subsidies and price controls.
政府干预市场是为了纠正市场失灵、再分配收入或实现更广泛的社会目标。常见的工具包括税收、补贴和价格管制。
ftwards by the amount of the tax per unit. New supply price = original supply price + tax.
间接税（如消费税）：使供给曲线向左移动，移动幅度为每单位税额。新的供给价格 = 原供给价格 + 税收。
ering the cost of production. New supply price = original supply price − subsidy per unit.
补贴：使供给曲线向右移动，降低生产成本。新的供给价格 = 原供给价格 − 每单位补贴。 Published by TutorHao | Year 9 Economics Revision Series | aleveler.com
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