📚 Year 9 SQA Economics: Formula & Theorem Quick-Reference Handbook | Year 9 SQA 经济：公式定理速查手册
This handbook provides a clear and concise reference for the essential formulas and economic theorems you need to master in Year 9 SQA Economics. It covers scarcity, opportunity cost, demand and supply, elasticity, market equilibrium, and government intervention, helping you build a solid foundation for exam success.
本手册为你提供清晰简明的 Year 9 SQA 经济必备公式和定理速查，涵盖稀缺性、机会成本、供求关系、弹性、市场均衡以及政府干预等核心主题，帮助你打下扎实基础，自信应对考试。
1. Scarcity and Choice | 稀缺性与选择
Scarcity means that resources are limited but human wants are unlimited. Because of scarcity, individuals, firms, and governments must make choices about how to use resources.
稀缺性意味着资源是有限的，而人类的欲望是无限的。由于稀缺性的存在，个人、企业和政府必须就资源的使用做出选择。
The fundamental economic problem arises from scarcity, forcing us to answer three questions: What to produce? How to produce? For whom to produce?
基本的经济问题正是源于稀缺性，它迫使我们回答三个问题：生产什么？如何生产？为谁生产？
2. Opportunity Cost | 机会成本
Opportunity cost is the value of the next best alternative that you give up when you make a choice. It is not all the alternatives you could have chosen, only the single next best one.
机会成本是指你做出选择时放弃的次优选项的价值。它不是所有可能选择的选项，而仅仅是次优的那一个。
Formula: Opportunity Cost = Value of the next best alternative forgone
公式：机会成本 = 所放弃的次优选项的价值
If a student chooses to study instead of working, the opportunity cost is the wages they could have earned from the job they gave up.
如果一名学生选择学习而不是打工，那么机会成本就是他们放弃的那份工作本可赚取的工资。
3. Production Possibility Frontier (PPF) | 生产可能性边界
The PPF shows the maximum combinations of two goods an economy can produce when all resources are fully and efficiently used. Points on the curve are efficient, inside the curve are inefficient, and outside are unattainable.
生产可能性边界（PPF）展示了一个经济体在所有资源得到充分有效利用时，能够生产的两种商品的最大组合。曲线上的点表示有效率的生产，曲线内的点表示无效率，曲线外的点则无法实现。
The slope of the PPF represents the opportunity cost of producing one more unit of the good on the horizontal axis. A straight-line PPF means constant opportunity cost; a bowed-out PPF means increasing opportunity cost.
PPF 的斜率代表多生产一单位横轴商品的机会成本。直线型 PPF 表示机会成本不变；向外弯曲的 PPF 表示机会成本递增。
4. Demand and 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 various prices over a given period. The law of demand states that, ceteris paribus (all other things being equal), as the price of a good rises, the quantity demanded decreases, and vice versa.
需求是指消费者在一定时期内，在各种价格水平下愿意且能够购买的商品或服务的数量。需求定律指出，在其他条件不变的情况下，商品价格上升，需求量下降；价格下降，需求量上升。
This inverse relationship between price and quantity demanded is shown by a downward-sloping demand curve. Factors that shift the entire demand curve include changes in income, tastes, prices of related goods, expectations, and the number of buyers.
价格与需求量之间这种反向关系表现为一条向下倾斜的需求曲线。引起整条需求曲线移动的因素包括收入变化、偏好、相关商品价格、预期和买家人数等。
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 various prices over a given period. The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied increases, and as price falls, the quantity supplied decreases.
供给是指生产者在各种价格水平下愿意且能够出售的商品或服务的数量。供给定律指出，在其他条件不变的情况下，商品价格上升，供给量增加；价格下降，供给量减少。
This positive relationship gives an upward-sloping supply curve. Shifts in the supply curve are caused by changes in production costs, technology, taxes and subsidies, number of sellers, and expectations.
这种正相关关系形成一条向上倾斜的供给曲线。供给曲线的移动由生产成本、技术、税收和补贴、卖家人数以及预期等因素的变化引起。
6. Market Equilibrium | 市场均衡
Market equilibrium occurs where the quantity demanded equals the quantity supplied. At this intersection, there is no shortage or surplus, and the market-clearing price is established.
市场均衡发生在需求量等于供给量时。在这个交点，既没有短缺也没有过剩，形成了市场出清价格。
If the price is above equilibrium, a surplus occurs, pushing the price down. If the price is below equilibrium, a shortage occurs, pushing the price up. This automatic adjustment is called the price mechanism.
如果价格高于均衡水平，就会出现过剩，促使价格下降。如果价格低于均衡水平，就会出现短缺，促使价格上升。这种自动调节过程称为价格机制。
7. 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 price. It is calculated using the formula:
需求价格弹性衡量需求量对价格变化的反应程度。其计算公式为：
PED = % Change in Quantity Demanded ÷ % Change in Price
PED = 需求量变动百分比 ÷ 价格变动百分比
If |PED| > 1, demand is price elastic; if |PED| < 1, demand is price inelastic; if |PED| nfluencing PED include the availability of substitutes, whether the good is a necessity or luxury, the proportion of income spent on it, and the time period considered.
如果 |PED| > 1，需求富有价格弹性；若 |PED| < 1，需求缺乏价格弹性；若 |PED| = 1，需求具有单位弹性。影响 PED 的因素包括替代品的可得性、商品是必需品还是奢侈品、支出占收入的比例以及所考虑的时间段。
8. Price Elasticity of Supply (PES) | 供给价格弹性
Price elasticity of supply measures the responsiveness of quantity supplied to a change in price. The formula is:
供给价格弹性衡量供给量对价格变化的反应程度。公式为：
PES = % Change in Quantity Supplied ÷ % Change in Price
PES = 供给量变动百分比 ÷ 价格变动百分比
ly is unit elastic. Key determinants are production time, availability of spare capacity, ease of storage, and level of inventories.
若 PES > 1，供给富有弹性；若 PES < 1，供给缺乏弹性；若 PES = 1，供给具有单位弹性。关键决定因素包括生产时间、闲置产能的可得性、储存难易程度以及库存水平。
9. Income Elasticity and Cross Elasticity of Demand | 需求收入弹性与交叉弹性
ity of demand (YED) measures how demand changes as consumer income changes. Formula: YED = % Change in Quantity Demanded ÷ % Income. A positive YED indicates a normal good; a negative YED indicates an inferior good.
需求收入弹性（YED）衡量需求如何随消费者收入变化而变化。公式：YED = 需求量变动百分比 ÷ 收入变动百分比。正的 YED 表示正常商品；负的 YED 表示低档商品。
ures how the demand for one good changes when the price of another changes. Formula: XED = % Change in Quantity Demanded of Good A ÷ % A positive XED means the goods are substitutes; a negative XED means they are complements.
需求交叉弹性（XED）衡量一种商品的需求如何随另一种商品价格的变化而变化。公式：XED = 商品 A 需求量变动百分比 ÷ 商品 B 价格变动百分比。正的 XED 表示两种商品互为替代品；负的 XED 表示它们互为互补品。
10. Total Revenue and Costs | 总收入与成本
ue = Price × Quantity sold. Average revenue is TR ÷ Quantity, which equals price per unit.
总收入（TR）是企业销售产品获得的货币总额。公式：总收入 = 价格 × 销量。平均收入 = 总收入 ÷ 销量，它等于每单位产品的价格。
nd variable costs (VC), which do change. Formula: Total Cost = Fixed Cost + Variable Cost.
总成本（TC）是生产中产生的所有成本之和，包括不随产量变化的固定成本（FC）和随产量变化的可变成本（VC）。公式：总成本 = 固定成本 + 可变成本。
- Average Cost (AC) = Total Cost ÷ Quantity
- 平均成本 (AC) = 总成本 ÷ 产量
- Marginal Cost (MC) = Change in Total Cost ÷ Change in Quantity
- 边际成本 (MC) = 总成本的变化量 ÷ 产量的变化量
11. Profit and Loss | 利润与亏损
al Revenue − Total Cost. If TR > TC, the firm makes a profit; if TR < TC, it makes a loss.
利润是总收入与总成本之间的差额。公式：利润 = 总收入 − 总成本。若 TR > TC，企业盈利；若 TR < TC，企业亏损。
in an industry; it occurs when TR = TC. Supernormal (abnormal) profit occurs when TR > TC.
正常利润是维持企业留在某行业所需的最低利润，此时 TR = TC。超常（经济）利润出现于 TR > TC 时。
12. Government Intervention: Taxes and Subsidies | 政府干预：税收与补贴
Indirect taxes (e.g., VAT) add to production costs, shifting the supply curve to the left. This raises the equilibrium price and reduces the equilibrium quantity. The tax burden is shared between consumers and producers depending on elasticities.
间接税（如增值税）会增加生产成本，使供给曲线向左移动，从而提高均衡价格，减少均衡数量。税负由消费者和生产者分担，分担比例取决于弹性。
uilibrium price and increases quantity. Formula for total subsidy cost to the government: Subsidy per unit × Quantity sold.
补贴是政府向生产者支付的款项，可降低生产成本，使供给曲线向右移动，从而降低均衡价格，增加均衡数量。政府的补贴总成本公式为：每单位补贴额 × 销量。
When evaluating government policies, you should consider the impact on consumers, producers, government revenue or expenditure, efficiency, and possible unintended consequences.
在评估政府政策时，你应当考虑对消费者、生产者、政府收入或支出、效率以及可能的意外后果的影响。
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