Microeconomics Core Curves | 微观经济学核心曲线梳理

📚 Microeconomics Core Curves | 微观经济学核心曲线梳理

In A-Level Economics, mastering the core curves of microeconomics is essential for tackling both multiple-choice and essay questions. These curves are not just lines on a graph—they encode the fundamental logic of market behaviour, costs, and welfare.

在 A-Level 经济学中，掌握微观经济学的核心曲线是应对选择题和论述题的关键。这些曲线不只是图形上的线条，它们承载着市场行为、成本与福利的基本逻辑，是经济推理的“语言”本身。

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1. The Demand Curve | 需求曲线

The demand curve shows the relationship between the price of a good and the quantity demanded, ceteris paribus. It slopes downward from left to right because of the law of diminishing marginal utility and the substitution and income effects.

需求曲线表示在其他条件不变的情况下，商品价格与需求量之间的关系。它向右下方倾斜，源于边际效用递减规律以及替代效应和收入效应。

• Movement along the curve: caused by a change in price.
• Shift of the curve: caused by changes in income, tastes, population, or prices of related goods.
• 向下移动与沿着曲线移动：价格变化 → 点的移动；非价格因素 → 整条曲线的平移。

When price falls from P₁ to P₂, quantity demanded rises from Q₁ to Q₂—this is shown as a movement from point A to point B along the same demand curve.

当价格从 P₁ 下降到 P₂ 时，需求量从 Q₁ 增加到 Q₂——这表现为需求曲线上的点从 A 移动到 B。

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2. The Supply Curve | 供给曲线

The supply curve shows the relationship between price and quantity supplied, ceteris paribus. It slopes upward because higher prices create greater incentive for firms to produce more.

供给曲线表示在其他条件不变时，价格与供给量之间的关系。它向右上方倾斜，因为价格越高，企业越有动力增加产量。

Key factors that shift the supply curve include: changes in costs of production, technology, taxes, subsidies, and the entry or exit of firms in the market.

使供给曲线移动的关键因素包括：生产成本、技术、税收、补贴以及市场中企业的进入或退出。

Supply shift factor	Direction
Better technology | 技术改进	Right | 右移
Higher wages | 工资上升	Left | 左移
Subsidy | 补贴增加	Right | 右移

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3. Market Equilibrium | 市场均衡

Equilibrium occurs where the supply and demand curves intersect. At the equilibrium price, the quantity demanded equals the quantity supplied, and there is no tendency for change.

均衡发生在供给曲线与需求曲线的交点处。在均衡价格下，需求量等于供给量，市场没有进一步变化的趋势。

Qd = Qs → Equilibrium price P* and quantity Q*

If price is above equilibrium, excess supply—a surplus—forces the price down. If price is below equilibrium, excess demand—a shortage—forces the price up. This is the market mechanism in action.

若价格高于均衡水平，便会形成超额供给（过剩），价格被迫下降；若价格低于均衡水平，则会出现超额需求（短缺），价格被迫上升。这正是市场机制的运作方式。

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4. Price Elasticity of Demand (PED) | 需求价格弹性

PED measures how responsive quantity demanded is to a change in price.

需求价格弹性衡量的是需求量对价格变化的敏感程度。

PED = % Δ Quantity Demanded ÷ % Δ Price

If PED > 1, demand is elastic. If PED < 1, demand is inelastic. Along a straight-line demand curve, elasticity varies: elastic at higher prices and inelastic at lower prices.

若 PED > 1，需求富有弹性；若 PED < 1，需求缺乏弹性。在一条线性需求曲线上，弹性是变化的：价格较高时富有弹性，价格较低时缺乏弹性。

Total Revenue = Price × Quantity

Firms use PED to decide on pricing strategy. If demand is elastic, a price cut increases total revenue; if inelastic, a price rise increases total revenue.

企业会依据 PED 制定价格策略：需求富有弹性时，降价会增加总收益；需求缺乏弹性时，涨价反而会增加总收益。

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5. Cross Elasticity of Demand (XED) | 需求交叉弹性

XED measures the responsiveness of demand for good X when the price of good Y changes.

交叉弹性衡量的是当商品 Y 的价格变化时，商品 X 的需求量所产生的反应程度。

XED = % Δ Demand for X ÷ % Δ Price of Y

• Positive XED: substitutes | 正值表示替代品
• Negative XED: complements | 负值表示互补品
• Zero XED: unrelated goods | 零值表示不相关

This cross-elasticity analysis helps firms understand competitive structures and helps governments in merger policy.

交叉弹性分析能帮助企业理解竞争格局，也能帮助政府评估兼并是否削弱市场竞争。

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6. Consumer Surplus | 消费者剩余

Consumer surplus is the difference between the maximum price consumers are willing to pay and the price they actually pay. It is shown in a supply–demand diagram as the triangular area below the demand curve and above the equilibrium price.

消费者剩余是消费者愿意支付的最高价格与实际支付价格之间的差额。在供需图中，它是需求曲线之下、均衡价格之上的三角形区域。

Consumer Surplus = Willingness to Pay − Actual Price

When a technological improvement shifts supply right, price falls, and consumer surplus expands—reflecting a welfare gain for buyers.

当技术进步使供给曲线右移时，价格下降，消费者剩余增加——这表现为买方福利的改善。

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7. Producer Surplus | 生产者剩余

Producer surplus is the difference between the price a producer actually receives and the minimum price at which it would be willing to sell. It is shown as the area above the supply curve and below the equilibrium price.

生产者剩余是生产者实际获得的价格与其愿意接受的最低价格之间的差额，在图中表现为供给曲线之上、均衡价格之下的区域。

Producer Surplus = Actual Price − Minimum Acceptable Price

Higher market prices generally increase producer surplus, improving producer welfare, while lower prices shrink it.

市场价格提高通常会增加生产者剩余，进而改善生产者福利；价格下降则会使其缩减。

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8. Total Cost Curves: TC, TFC, TVC | 总成本曲线

Total fixed cost (TFC) is constant regardless of output. Total variable cost (TVC) rises as output increases. Total cost (TC) is the vertical sum of TFC and TVC.

总固定成本（TFC）在产量变化时保持不变；总可变成本（TVC）随着产量增加而上升；总成本（TC）是 TFC 与 TVC 的垂直加总。

Output	0	1	2	3
TFC	10	10	10	10
TVC	0	6	11	18
TC	10	16	21	28

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9. Average and Marginal Cost Curves | 平均成本与边际成本曲线

Average cost (AC) is TC divided by output. Marginal cost (MC) is the addition to total cost from producing one more unit. Both curves are U-shaped in the short run due to the law of diminishing returns.

平均成本（AC）等于 TC 除以产量；边际成本（MC）是每增加一单位产量所增加的总成本。在短期内，由于边际报酬递减规律的影响，两者都是 U 形曲线。

AC = TC ÷ Q MC = ΔTC ÷ ΔQ

When MC is below AC, AC falls; when MC is above AC, AC rises. Therefore, MC crosses AC at the minimum point of AC—an essential relationship in cost diagrams.

当 MC 低于 AC 时，AC 下降；当 MC 高于 AC 时，AC 上升。因此，MC 必定穿过 AC 的最低点——这是成本图分析中至关重的关系。

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10. Marginal and Average Revenue Curves | 边际收益与平均收益曲线

In perfect competition, a firm is a price taker, so AR and MR are horizontal lines at the market price. In monopoly, the demand curve is the AR curve, which slopes downward, and MR lies below AR.

在完全竞争市场下，企业是价格接受者，AR 与 MR 都是位于市场价格处的水平直线；在垄断市场下，需求曲线即 AR 曲线，向右下方倾斜，且 MR 位于 AR 下方。

AR = TR ÷ Q MR = ΔTR ÷ ΔQ

The reason MR is below AR in monopoly is that to sell additional units, the firm must lower the price on all units sold, reducing the revenue gained from the extra unit.

垄断市场中 MR 低于 AR 的原因在于：为了卖出额外一单位产量，企业必须降低所有单位的价格，从而削减了每增加一单位产量所能带来的收益。

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11. Profit Maximisation: MC = MR | 利润最大化：MC＝MR

The profit-maximising output for any firm is where marginal cost equals marginal revenue, provided price exceeds average variable cost. This is the golden rule of microeconomic theory.

任何企业的利润最大化产量都出现在边际成本等于边际收益之处，前提是价格高于平均可变成本。这是微观经济理论中的黄金法则。

MC = MR → Profit Maximising Output Q*

Supernormal profit is shown by the rectangle: (AR − AC) × Q, where AR is price and AC is average cost at the chosen output.

超额利润可表示为矩形面积：(AR − AC) × Q，其中 AR 即价格，AC 为选定产出下的平均成本。

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12. Shutdown Point and Break-Even Point | 停业点与盈亏平衡点

The break-even point occurs where average revenue equals average cost (AR = AC), meaning zero economic profit. The shutdown point in the short run is where price equals minimum average variable cost (P = min AVC).

盈亏平衡点出现在平均收益等于平均成本（AR = AC）时，此时经济利润为零。短期停业点则是价格等于最低平均可变成本（P = min AVC）的位置。

If price falls below the shutdown point, the firm loses more by staying open than by closing; if price lies between minimum AVC and minimum AC, the firm should continue producing in the short run to minimise losses.

如果价格低于停业点，继续经营比停产亏损更大；如果价格介于最低 AVC 与最低 AC 之间，企业在短期内仍应继续生产，以尽量减少亏损。

Break-even: P = AC Shutdown: P = AVC

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Understanding these core curves—demand, supply, cost, revenue, and welfare—will give you a powerful framework for analysing market behaviour and policy impacts in A-Level Economics examinations. Draw them clearly, label every axis and curve, and explain each shift with real-world examples.

理解这些核心曲线——需求、供给、成本、收益与福利曲线——将为你分析 A-Level 经济学考试中的市场行为和政策影响提供强大的分析框架。请务必清晰作图、标注所有坐标轴与曲线名称，并结合现实案例解释每一次曲线移动背后的经济学含义。

Published by TutorHao | Economics Revision Series | aleveler.com

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