📚 Year 9 SQA Economics: Quick-Reference Formula Handbook | Year 9 SQA 经济：公式定理速查手册
Welcome to your one-stop handbook for the essential formulas and economic laws in Year 9 SQA Economics. This guide breaks down every key concept with clear definitions and paired bilingual explanations, helping you master the foundations of the subject and build the confidence to tackle exam-style questions efficiently. Whether you are revising scarcity, elasticity or profit calculations, you will find all the must-know relationships in one place.
欢迎使用这份 Year 9 SQA 经济必备公式与定理速查手册。本指南用清晰的定义和中英双语对照的方式拆解每一个关键概念，帮助你扎实掌握学科基础，从容应对考试题型。无论是复习稀缺性、弹性还是利润计算，这里汇集了你必须掌握的所有核心关系。
1. Scarcity and Choice | 稀缺性与选择
Economics begins with the fundamental problem of scarcity. Our wants are unlimited, but the resources available to satisfy them – land, labour, capital and enterprise – are finite. Because scarcity exists, individuals, firms and governments must make choices about how to allocate these limited resources, and every choice involves giving something up.
经济学始于稀缺性这一基本问题。我们的欲望是无限的，但用来满足这些欲望的资源——土地、劳动力、资本和企业才能——是有限的。由于稀缺性的存在，个人、企业和政府必须就如何分配这些有限资源做出选择，而每一个选择都意味着要放弃某些东西。
Economists use the concept of trade-offs to describe what is sacrificed when a decision is made. For instance, a student who spends an extra hour studying economics rather than playing video games faces a trade-off between higher grades and leisure time. This leads directly to the definition of opportunity cost, which we explore in a later section.
经济学家用权衡取舍来描述当做出一个决定时所牺牲的东西。例如，一个学生多花一小时学习经济而不是打游戏，就面临着更高成绩与休闲时间之间的权衡。这直接引出了机会成本的定义，我们将在后面章节探讨。
2. Demand and the Law of Demand | 需求与需求定律
Demand refers to the quantity of a good or service that consumers are willing and able to purchase at various prices over a given period. The Law of Demand states that, ceteris paribus (all other factors held constant), as the price of a product rises, the quantity demanded falls, and vice versa. This inverse relationship gives the demand curve its downward slope.
需求指的是在一定时期内，消费者在不同价格水平下愿意并且能够购买的商品或服务数量。需求定律指出，在其他条件不变的前提下，当产品价格上升时，需求量下降；反之亦然。这种反向关系使得需求曲线向右下方倾斜。
A typical demand schedule shows, for example, that at a price of £2 consumers demand 80 units, but at £5 they demand only 30 units. The relationship can be simplified into a demand function, but at Year 9 level it is enough to recognise the negative correlation and be able to move along a demand curve when the price changes.
一个典型的需求表显示，例如价格为2英镑时消费者需求80单位，但价格上升到5英镑时需求只有30单位。这种关系可以简化为需求函数，但在Year 9阶段，只需要认识到价格与需求量之间的负相关关系，并能在价格变动时沿着需求曲线移动即可。
3. Supply and the Law of Supply | 供给与供给定律
Supply is the quantity of a product that producers are willing and able to offer for sale at each price over a period of time. The Law of Supply asserts a direct relationship: as the market price rises, the quantity supplied increases, ceteris paribus. Higher prices create a profit incentive for firms to expand production, so the supply curve slopes upward.
供给是指在一段时间内，生产者在各个价格水平下愿意并且能够出售的产品数量。供给定律指出一种正向关系：在其他条件不变的情况下，市场价格上升，供给量增加。更高的价格为厂商扩大生产提供了利润激励，因此供给曲线向右上方倾斜。
For instance, a farmer might supply 200 kg of strawberries at £3 per kg but increase supply to 500 kg when the price reaches £5 per kg. Remember that movements along the supply curve are caused only by changes in price, while shifts of the whole curve occur when other factors like technology or input costs change.
例如，一个农民在每公斤草莓售价3英镑时供应200公斤，但在价格达到每公斤5英镑时把供应量增加到500公斤。记住，只有当价格变化时我们才会沿着供给曲线移动；当技术或投入成本等其他因素变化时，整条供给曲线会发生位移。
4. Market Equilibrium | 市场均衡
Market equilibrium occurs at the price where 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 exactly. The equilibrium condition is:
市场均衡发生在需求量等于供给量的价格水平上。此时价格没有变动的趋势，因为买方和卖方的计划正好一致。均衡条件为：
Qd = Qs
If the prevailing price is above equilibrium, a surplus (excess supply) emerges, putting downward pressure on price. If the price is below equilibrium, a shortage (excess demand) drives the price upward. The market gravitates towards equilibrium over time, although real-world shocks can cause temporary disequilibrium.
如果当前市场价格高于均衡价格，就会出现过剩（超额供给），给价格带来下行压力。如果价格低于均衡价格，就会出现短缺（超额需求），推动价格上升。虽然现实中的冲击可能引起暂时的不均衡，但市场会逐步向均衡回归。
Understanding equilibrium is crucial because it helps explain why prices change. A rise in demand (rightward shift of the demand curve) simultaneously increases equilibrium price and quantity, while a rise in supply (rightward shift of the supply curve) lowers price but raises the equilibrium quantity.
理解均衡至关重要，因为它有助于解释价格为何变动。需求增加（需求曲线右移）会同时提高均衡价格和均衡数量，而供给增加（供给曲线右移）会降低均衡价格但提高均衡数量。
5. Total Revenue (TR) | 总收入
Total revenue is the income a firm receives from selling its goods or services. It is calculated simply as the price per unit multiplied by the quantity sold. The formula is one of the most frequently used in economics:
总收入是企业通过销售商品或服务所获得的全部收入。它的计算非常简单，就是单位售价乘以销售数量。这个公式是经济学中最常用的公式之一：
TR = P × Q
where P is price and Q is quantity. For example, if a firm sells 150 units of a product at £8 each, total revenue equals £1,200. Revenue is a key indicator of a firm's performance and directly affects profit calculations.
其中 P 代表价格，Q 代表数量。例如，一家企业以每件8英镑的价格售出150件产品，总收入就是1200英镑。收入是衡量企业业绩的关键指标，并直接影响利润的计算。
When analysing how price changes affect total revenue, the concept of price elasticity of demand becomes essential. If demand is elastic, a price cut raises total revenue; if demand is inelastic, a price increase raises total revenue. This link is explored in the next section.
在分析价格变动如何影响总收入时，需求价格弹性这一概念就变得至关重要。如果需求富有弹性，降价会增加总收入；如果需求缺乏弹性，涨价会增加总收入。这一联系将在下一节中探讨。
6. 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. The basic formula (ignoring the negative sign for simplicity) is:
需求价格弹性衡量需求量对价格变动的反应程度。为简化起见，基本公式（忽略负号）为：
PED = % change in quantity demanded / % change in price = (%ΔQd) / (%ΔP)
The coefficient tells us how elastic or inelastic the demand is. The table below summarises the key classifications, combining English descriptions with Chinese equivalents for easy reference.
该系数告诉我们需求是富有弹性还是缺乏弹性。下表汇总了主要的分类方式，并结合英文描述和中文说明，方便查阅。
| Numerical value | English description | 数值 | 中文描述 |
|---|---|---|---|
| PED > 1 | Elastic demand | PED > 1 | 富有弹性 |
| PED < 1 | Inelastic demand | PED < 1 | 缺乏弹性 |
| PED = 1 | Unit elastic demand | PED = 1 | 单位弹性 |
| PED = 0 | Perfectly inelastic demand | PED = 0 | 完全无弹性 |
| PED = ∞ | Perfectly elastic demand | PED = ∞ | 完全弹性 |
Knowledge of PED is vital for business pricing strategies and government taxation policies. Products with many close substitutes, like cola drinks, typically have elastic demand, whereas necessities such as basic food items tend to have inelastic demand.
掌握 PED 的知识对于企业定价策略和政府税收政策至关重要。拥有众多相近替代品的产品，如可乐饮料，通常富有弹性；而像基本食品这样的必需品则往往缺乏弹性。
7. 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. The formula is similar to PED, but focuses on the producer side:
供给价格弹性衡量供给量对价格变动的反应程度。其公式与 PED 相似，但聚焦于生产者一侧：
PES = % change in quantity supplied / % change in price = (%ΔQs) / (%ΔP)
Supply is described as elastic when PES > 1, meaning firms can adjust production quickly in response to price changes. Supply is inelastic when PES < 1, suggesting production cannot be increased easily in the short run, perhaps because of time lags or limited capacity.
当 PES > 1 时，供给富有弹性，意味着企业能迅速根据价格变化调整产量。当 PES < 1 时，供给缺乏弹性，表示在短期内生产不易扩大，可能是由于时间滞后或产能有限。
Key determinants of PES include the time period under consideration, the availability of stocks, the complexity of the production process and the extent to which spare capacity exists. Agricultural products often exhibit inelastic supply in the short run because planting cycles cannot be accelerated, whereas manufactured goods may have more elastic supply.
影响 PES 的关键因素包括所考虑的时间周期、库存的可得性、生产过程的复杂程度以及是否存在闲置产能。农产品在短期内常表现出缺乏弹性的供给，因为种植周期无法加速；而工业制成品则可能具有更富有弹性的供给。
8. Costs of Production | 生产成本
To make sound business decisions, firms need to understand their costs. The most basic distinction is between fixed costs (FC), which do not vary with output in the short run (e.g. rent, insurance), and variable costs (VC), which change directly with the level of production (e.g. raw materials, hourly wages). The total cost is the sum of the two:
企业要做出合理的经营决策，就需要了解自身的成本。最基本的区分是固定成本 (FC)和可变成本 (VC)。固定成本在短期内不随产量变化（如租金、保险费），可变成本则直接随生产水平变化（如原材料、小时工资）。总成本是二者之和：
TC = FC + VC
It is also useful to express costs on a per-unit basis. Average total cost (ATC) can calculate average fixed cost (AFC = FC ÷ Q) and average variable cost (AVC = VC ÷ Q):
以每单位为基础表达成本也很有用。平均总成本 (ATC)等于总成本除以产量。同样地，我们可以计算平均固定成本 (AFC = FC ÷ Q) 和平均可变成本 (AVC = VC ÷ Q)：
ATC = TC ÷ Q
AFC = FC ÷ Q
AVC = VC ÷ Q
If a bakery pays £800 in fixed costs and £4 in variable costs for each cake, producing 200 cakes gives TC = 800 + (4 × 200) = £1,600. Then ATC = 1600 ÷ 200 = £8 per cake. These average cost figures help a business set a profitable selling price.
假如一家面包店支付800英镑固定成本，每个蛋糕的可变成本为4英镑，则生产200个蛋糕时 TC = 800 + (4×200) = 1600英镑。那么 ATC = 1600 ÷ 200 = 每个蛋糕8英镑。这些平均成本数据有助于企业确定有利可图的售价。
9. Profit and Loss | 利润与亏损
Profit is the reward for risk-taking and the ultimate goal of most private-sector firms. It is defined as the difference between total revenue and total cost. The formula is straightforward:
利润是对承担风险的回报，也是大多数私营企业追求的最终目标。它被定义为总收入与总成本之间的差额。公式简单明了：
Profit = TR − TC
When TR exceeds TC, the firm makes a supernormal (economic) profit. If TR equals TC, the firm is said to be breaking even – it earns just enough to cover all its costs, including the opportunity cost of the entrepreneur's time and capital. If TR falls below TC, the firm incurs a loss.
当 TR 大于 TC 时，企业获得超额（经济）利润。如果 TR 等于 TC，企业则处于盈亏平衡——它刚好能够覆盖所有成本，包括企业家时间和资本的机会成本。如果 TR 低于 TC，企业则出现亏损。
For example, suppose our bakery sells 200 cakes at £10 each, generating TR = £2,000. With TC of £1,600, profit equals £ siness performance and predict how changes in costs or price might affect the bottom line.
例如，假设我们的面包店以每个10英镑的价格售出200个蛋糕，TR = 2000英镑。当 TC 为1600英镑时，利润为400英镑。清楚地掌握利润公式有助于学生快速评估企业绩效，并预测成本或价格变化将如何影响最终盈亏。
10. Opportunity Cost | 机会成本
Opportunity cost is one of the most powerful concepts in economics. It represents the value of the next best alternative that must be forgone when a choice is made. Although it is not expressed as a single numerical formula, the principle can be stated as:
机会成本是经济学中最强有力的概念之一。它代表着当做出一个选择时，所必须放弃的次优替代选项的价值。尽管它无法用一个单一的数字公式表示，但这一定理可以表述为：
Opportunity cost = value of the next best alternative sacrificed
机会成本 = 所牺牲的次优选项的价值
Every decision, from a government building a new hospital instead of a motorway to a student choosing economics over history, carries an opportunity cost. The cost is not the total of all alternatives, only the single best alternative given up. Understanding this helps individuals and societies allocate scarce resources more efficiently.
每一个决策，无论是政府选择建一家新医院而不是一条高速公路，还是学生选择学习经济而非历史，都伴随着机会成本。成本并非所有替代选项的总和，而只是所放弃的单一最佳选项。理解这一点有助于个人和社会更有效地配置稀缺资源。
When a country uses its farmland to grow wheat, the opportunity cost might be the corn it could have produced instead. By constantly asking "what else could we have done with these resources?", economists highlight the real cost of any action.
当一个国家利用其农田种植小麦时，机会成本可能是它本可以生产的玉米。经济学家通过不断追问“我们原本可以用这些资源做些什么？”，凸显了任何行动的真实成本。
11. Government Intervention: Indirect Taxes and Subsidies | 政府干预：间接税与补贴
Governments often intervene in markets to correct externalities or generate revenue. An indirect tax (e.g. VAT, excise duty) is a charge on a good or service that shifts the supply curve to the left. The new supply curve is vertically higher by the amount of the tax. If a unit tax t is imposed, the new supply price is:
政府经常干预市场以纠正外部性或创造财政收入。间接税（如增值税、消费税）是对商品或服务征收的费用，它会使供给曲线向左移动。新的供给曲线垂直向上移动的幅度等于税额。如果征收每单位税额 t，则新的供给价格为：
Prices( after tax) = Prices( before) + t
The economic burden of the tax is shared between consumers (who pay a higher price) and producers (who receive a lower net price). The exact split depends on the relative elasticities of demand and supply – the more inelastic side bears a larger share.
税收的经济负担由消费者（支付更高价格）和生产者（获得更低的净价格）共同承担。具体的分担比例取决于需求和供给的相对弹性——更缺乏弹性的一方会承担更大份额。
A subsidy is a payment from the government to producers that lowers their costs and shifts the supply curve to the right. With a per-unit subsidy s, the new supply price is:
补贴是政府向生产者提供的支付，用以降低其成本，并使供给曲线向右移动。在每单位补贴额为 s 的情况下，新的供给价格为：
Prices( after subsidy) = Prices( before) − s
Subsidies are often used to encourage production of goods with positive externalities, such as renewable energy or education.
Published by TutorHao | Year 9 Economics Revision Series | aleveler.com
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