📚 Year 10 SQA Economics: Quick Reference Handbook of Formulas and Theorems | Year 10 SQA 经济：公式定理速查手册
This handbook gathers the essential formulas, definitions and theorems you need to master for Year 10 SQA Economics. Each entry is explained in plain English and Chinese to strengthen your understanding and help you apply these tools confidently in assessments. Use it for quick revision, homework support or last-minute drilling before tests.
本手册汇集了 Year 10 SQA 经济学科必须掌握的核心公式、定义与定理。每个条目均用清晰的中英文解释，以加深理解，帮助你在考核中自信地运用这些工具。无论是快速复习、作业支持还是考前冲刺，它都是你的得力助手。
1. Price Elasticity of Demand (PED) | 需求价格弹性 (PED)
Price Elasticity of Demand (PED) measures the responsiveness of the quantity demanded of a good to a change in its own price. It is always negative because price and quantity demanded move in opposite directions, but economists usually work with the absolute value.
需求价格弹性 (PED) 衡量商品的需求量对自身价格变动的反应程度。由于价格与需求量呈反向变动，PED 总为负值，但经济学家通常使用其绝对值进行分析。
PED = %ΔQd ÷ %ΔP
If |PED| s elastic: a small price change causes a larger percentage change in quantity demanded. If |PED| < 1, demand is inelastic: the quantity demanded is relatively unresponsive. If |PED| = 1, demand is unit elastic. Perfectly elastic demand (|PED| rs when any price increase reduces quantity demanded to zero; perfectly inelastic demand (|PED| good is a necessity or luxury, the share of income spent, and the time period considered.
若 |PED| > 1，需求富有弹性：价格微小变动会引起需求量更大比例变动。若 |PED| < 1，需求缺乏弹性：需求量相对不敏感。若 |PED| = 1，需求为单位弹性。完全弹性需求 (|PED| = ∞) 意味着价格任何上升都将使需求量降为零；完全无弹性需求 (|PED| = 0) 表明需求量完全不随价格变动。主要影响因素包括替代品的可得性、商品属于必需品还是奢侈品、支出占收入的比例以及所考虑的时间长度。
2. Income Elasticity of Demand (YED) | 需求收入弹性 (YED)
Income Elasticity of Demand (YED) indicates how sensitive the demand for a good is to changes in consumers' income. It allows us to classify goods as normal or inferior.
需求收入弹性 (YED) 表示商品需求对消费者收入变动的敏感程度。它帮助我们区分正常品和低档品。
YED = %ΔQd ÷ %ΔY
If YED > 0, the good is a normal good (demand rises as income rises). Among normal goods, if YED > 1, it is a luxury good; if 0 < YED < 1, it is a necessity. If YED < 0, the good is an inferior good: an increase in income leads to a fall in demand as consumers switch to higher-quality alternatives. Firms use YED to forecast the impact of economic growth on their sales.
若 YED > 0，该商品为正常品（收入增加，需求上升）。在正常品中，若 YED > 1，则为奢侈品；若 0 < YED < 1，则为必需品。若 YED < 0，该商品为低档品：收入增长会导致需求下降，因为消费者转向更优质的替代品。企业利用 YED 预测经济增长对销售的影响。
3. Cross Elasticity of Demand (XED) | 需求交叉弹性 (XED)
Cross Elasticity of Demand (XED) measures the responsiveness of demand for one good (A) when the price of another good (B) changes. This determines whether goods are substitutes or complements.
需求交叉弹性 (XED) 衡量一种商品 (A) 的需求量对另一种商品 (B) 价格变动的反应程度，用以判断商品间是替代关系还是互补关系。
XED = %ΔQd of good A ÷ %ΔP of good B
of B decreases the demand for A). If XED = 0, the goods are unrelated. The larger the absolute value, the stronger the relationship. Businesses monitor XED to anticipate the effects of competitors' pricing strategies and to identify close substitutes or complementary products.
若 XED > 0，两种商品互为替代品（B 的价格上升导致 A 的需求增加）。若 XED < 0，它们互为互补品（B 的价格上升导致 A 的需求减少）。若 XED = 0，则两种商品不相关。绝对值越大，关系越强。企业通过监测 XED 来预测竞争对手定价策略的影响，并识别紧密的替代品或互补产品。
4. Price Elasticity of Supply (PES) | 供给价格弹性 (PES)
Price Elasticity of Supply (PES) shows how much the quantity supplied of a good changes in response to a change in its price. It is always positive due to the law of supply.
供给价格弹性 (PES) 表示商品的供给量对自身价格变动的反应程度。由于供给定律，该值总为正。
PES = %ΔQs ÷ %ΔP
cates perfectly inelastic supply; PES = ∞ describes perfectly elastic supply. Determinants include the length of the production period, the availability of spare capacity, the ease of switching resources between uses, the ability to store stocks, and the time frame under consideration. In the short run, supply tends to be more inelastic; in the long run, firms can adjust all factors, making supply more elastic.
若 PES > 1，供给富有弹性（生产者能迅速增加产出）。若 PES < 1，供给缺乏弹性（产出反应缓慢）。PES = 0 表示完全无弹性供给；PES = ∞ 为完全弹性供给。决定因素包括生产周期的长短、闲置产能的可得性、资源在不同用途间转换的难易程度、储存能力以及所考虑的时间范围。短期内供给通常更缺乏弹性；长期内企业可调整所有要素，使供给变得更有弹性。
5. Revenue: Total, Average and Marginal | 收益：总收益、平均收益与边际收益
Revenue is the income a firm receives from selling its output. Understanding the relationships between total, average and marginal revenue is crucial for pricing and output decisions.
收益是企业通过销售产出获得的收入。理解总收益、平均收益与边际收益之间的关系对定价和产量决策至关重要。
TR = P × Q
AR = TR ÷ Q = P
MR = ΔTR ÷ ΔQ
urve is horizontal. In imperfectly competitive markets (e.g. monopoly), the firm faces a downward-sloping demand curve. To sell an extra unit, the price must be lowered on all units, so MR lies below AR and can become negative. The PED of the demand curve directly affects the relationship between price changes and total revenue: if demand is elastic, a price cut raises TR; if demand is inelastic, a price cut lowers TR.
在完全竞争市场中，企业是价格接受者：AR = MR = P，因此需求曲线为水平线。在不完全竞争市场（如垄断）中，企业面临向下倾斜的需求曲线。为了多销售一单位产品，必须降低所有单位的价格，因此 MR 位于 AR 下方，且可能变为负值。需求曲线的 PED 直接影响价格变动与总收益的关系：若需求富有弹性，降价会增加总收益；若需求缺乏弹性，降价则会降低总收益。
6. Costs: Total, Average and Marginal | 成本：总成本、平均成本与边际成本
A firm's costs are split into fixed costs (TFC), which do not vary with output in the short run, and variable costs (TVC), which change with the level of production. These combine to form total cost (TC).
企业的成本分为固定成本 (TFC) 和可变成本 (TVC)。固定成本在短期内不随产量变化，可变成本则随生产水平变动。两者相加构成总成本 (TC)。
TC = TFC + TVC
ATC = TC ÷ Q
AFC = TFC ÷ Q
AVC = TVC ÷ Q
MC = ΔTC ÷ ΔQ
Average total cost (ATC) is also known as unit cost. AFC falls continuously as output rises (spreading overheads). AVC eventually rises due to diminishing marginal returns. The ATC curve is U-shaped. Marginal cost (MC) is the extra cost of producing one more unit; it cuts both AVC and ATC at their minimum points. Short-run cost curves are vital for determining a firm's supply decision and break-even price.
平均总成本 (ATC) 也称单位成本。随着产量增加，AFC 持续下降（分摊固定费用）。由于边际收益递减，AVC 最终上升。ATC 曲线呈 U 形。边际成本 (MC) 是额外生产一单位产品所增加的成本；它通过 AVC 和 ATC 的最低点。短期成本曲线对于确定企业的供给决策和盈亏平衡价格至关重要。
7. Profit Maximisation Rule | 利润最大化法则
Economists assume that firms aim to maximise profit. The profit-maximising level of output occurs where the additional revenue from selling one more unit equals the additional cost of producing it.
经济学家假设企业以利润最大化为目标。当额外销售一单位产品所带来的收益等于生产它的额外成本时，即达到利润最大化的产量水平。
MR = MC
duces profit, so the firm should cut back. Only when MR = MC is profit maximised. This rule applies to all market structures. At the profit-maximising output, total profit can be calculated as (AR - ATC) × Q, where (AR - ATC) is profit per unit. The firm will only produce in the short run if price covers AVC; otherwise it will shut down.
若 MR > MC，额外一单位带来的收益大于成本，扩大产量可增加利润。若 MR < MC，再多生产一单位将减少利润，企业应缩减产量。只有当 MR = MC 时，利润达到最大。这一法则适用于所有市场结构。在利润最大化产量下，总利润可表示为 (AR - ATC) × Q，其中 (AR - ATC) 为单位利润。短期内，只有当价格能弥补 AVC 时企业才会生产，否则将停产。
8. Opportunity Cost and the Production Possibility Frontier | 机会成本与生产可能性边界
Opportunity cost is the value of the next best alternative forgone when a choice is made. It is a fundamental concept because resources are scarce. The Production Possibility Frontier (PPF) illustrates opportunity costs, trade-offs and efficiency for an economy producing two goods.
机会成本是做出选择时所放弃的次优替代品的价值。由于资源稀缺，这是一个基础概念。生产可能性边界 (PPF) 展示了生产两种商品的经济体所面临的机会成本、权衡取舍与效率。
The PPF shows the maximum combinations of output attainable with existing resources and technology. Points on the curve are productively efficient; points inside indicate under-employment of resources; points outside are unattainable. The slope of the PPF represents the marginal rate of transformation (MRT) — the opportunity cost of one good in terms of the other. A straight-line PPF implies constant opportunity cost; a concave (bowed-out) PPF reflects increasing opportunity cost as resources are not equally suited to producing both goods. Economic growth shifts the PPF outward.
PPF 展示了现有资源和技术条件下可达到的最大产出组合。曲线上的点表示生产有效；曲线内的点表示资源未充分利用；曲线外的点无法达到。PPF 的斜率代表边际转换率 (MRT)——一种商品用另一种商品表示的机会成本。直线 PPF 意味着机会成本不变；凹向原点的 PPF 反映机会成本递增，因为资源并非同等适合生产两种商品。经济增长会使 PPF 向外移动。
9. Comparative Advantage and Gains from Trade | 比较优势与贸易收益
Comparative advantage exists when a country can produce a good at a lower opportunity cost than another country. This principle explains why specialisation and trade can benefit all parties, even if one country is absolutely more efficient in producing everything.
当一国能以比另一国更低的机会成本生产某种商品时，就存在比较优势。该原理解释了为何即使一国在各方面都绝对更高效，专业化和贸易仍可使所有参与方获益。
To determine comparative advantage, compare opportunity cost ratios. For example, if Country X gives up 2 units of cloth to produce 1 unit of wine, and Country Y gives up 3 units of cloth, Country X has the comparative advantage in wine. Each country should specialise in the good for which it has the lower opportunity cost and then trade. The terms of trade must lie between the two countries' opportunity cost ratios for both to gain. Trade increases total world output and allows consumption beyond individual PPFs.
判断比较优势，需比较机会成本比率。例如，若 X 国生产 1 单位酒需放弃 2 单位布，而 Y 国需放弃 3 单位布，则 X 国在酒的生产上具有比较优势。每个国家应专门生产其机会成本较低的商品并进行贸易。贸易条件必须处于两国机会成本比率之间，双方才能获益。贸易增加了世界总产出，使各国消费可以超越各自的 PPF。
10. Market Equilibrium, Surplus and Shortage | 市场均衡、过剩与短缺
Market equilibrium occurs where the quantity demanded equals the quantity supplied at a particular price. At this point, there is no tendency for the price to change — it is the market-clearing price.
市场均衡出现在某一价格下需求量等于供给量时。在该点，价格没有变动的趋势——即市场出清价格。
Qd = Qs
the price down toward equilibrium. If the price is below equilibrium, a shortage (excess demand) emerges. Consumers bid up the price, and it rises back toward equilibrium. These adjustments continue until Qd = Qs. The functions of the price mechanism — signalling, rationing and incentivising — valuating the effects of government interventions such as price ceilings and price floors.
若市场价格高于均衡水平，将出现过剩（超额供给）。生产者会降价清仓，促使价格回落至均衡。若价格低于均衡水平，则出现短缺（超额需求）。消费者竞相出价，推动价格回升至均衡。这一调整过程将持续至 Qd = Qs。价格机制的信号、配给和激励功能正是通过这些变动发挥作用。掌握均衡分析对于评估价格上限、价格下限等政府干预措施的效果至关重要。
Published by TutorHao | Economics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图