📚 Types of Cost, Revenue and Profit; Short-Run and Long-Run Production | 成本、收入与利润的类型；短期与长期生产
In Cambridge A-Level Economics, firms make production and pricing decisions by comparing costs, revenue and profit. The time horizon matters: in the short run at least one factor of production is fixed, while in the long run all factors can be varied. Understanding the shapes of cost and revenue curves helps explain how firms maximise profit and why long-run average costs may fall or rise.
在剑桥 A-Level 经济学中，企业通过比较成本、收入和利润来做出生产与定价决策。时间范围很关键：短期内至少有一种生产要素固定，长期内所有要素都可变动。理解成本与收入曲线的形状，有助于解释企业如何实现利润最大化，以及长期平均成本为何会下降或上升。
1. The Short Run and the Long Run | 短期与长期
In microeconomics, the short run is a period during which at least one factor of production is fixed. Typically capital and land are fixed, while labour and raw materials can be changed. The long run is a planning horizon in which all factors of production are variable. There is no fixed calendar length; a steel plant may have a longer short run than a coffee shop because it takes more time to change its capital equipment.
在微观经济学中，短期是指至少有一种生产要素固定不变的时期。通常资本和土地固定，而劳动力和原材料可以改变。长期则是一个所有生产要素都可变的规划期。短期没有固定的日历长度；钢铁厂的短期可能比咖啡店更长，因为更换资本设备需要更多时间。
Firms use the short run to make output decisions with existing capacity, and the long run to choose the scale of production. The distinction affects cost curves: short-run costs include fixed costs, while long-run costs are all variable.
企业利用短期在现有产能下做出产量决策，利用长期选择生产规模。这一区分影响成本曲线：短期成本包含固定成本，而长期成本都是可变的。
2. Fixed Costs, Variable Costs and Total Costs | 固定成本、可变成本与总成本
Fixed costs (FC) do not change with output in the short run. Examples include rent, insurance premiums and salaries of permanent staff. Variable costs (VC) change directly with output, such as raw materials, energy and piece-rate wages. Total cost (TC) is the sum of fixed and variable costs: TC = FC + VC.
固定成本（FC）在短期内不随产量变化。例如租金、保险费和长期员工的薪水。可变成本（VC）随产量直接变化，如原材料、能源和计件工资。总成本（TC）是固定成本与可变成本之和：TC = FC + VC。
Because FC is constant, an increase in output spreads fixed costs over more units, reducing average fixed cost. VC rises as more variable inputs are used, so total cost rises at an increasing rate once diminishing returns set in.
由于固定成本不变，产量增加会把固定成本分摊到更多单位，从而降低平均固定成本。随着更多可变投入被使用，可变成本上升，因此一旦边际收益递减开始，总成本会以递增速度上升。
3. Average Costs and Marginal Costs | 平均成本与边际成本
Average total cost (ATC or AC) is total cost divided by output: ATC = TC ÷ Q. Average fixed cost (AFC) is FC ÷ Q, and average variable cost (AVC) is VC ÷ Q. Marginal cost (MC) is the extra cost of producing one more unit: MC = ΔTC ÷ ΔQ.
平均总成本（ATC 或 AC）是总成本除以产量：ATC = TC ÷ Q。平均固定成本（AFC）为 FC ÷ Q，平均可变成本（AVC）为 VC ÷ Q。边际成本（MC）是多生产一单位产品所增加的成本：MC = ΔTC ÷ ΔQ。
MC cuts both AVC and ATC at their minimum points. When MC is below average cost, average cost falls; when MC is above average cost, average cost rises. This relationship is essential for understanding the U-shaped short-run cost curves.
边际成本曲线分别穿过平均可变成本曲线和平均总成本曲线的最低点。当边际成本低于平均成本时，平均成本下降；当边际成本高于平均成本时，平均成本上升。这一关系对理解 U 形短期成本曲线至关重要。
4. Short-Run Production: Total, Average and Marginal Product | 短期生产：总产量、平均产量与边际产量
In the short run, output depends on combining fixed capital with variable labour. Total product (TP) is total output. Average product (AP) is output per unit of labour: AP = TP ÷ L. Marginal product (MP) is the extra output from one additional worker: MP = ΔTP ÷ ΔL.
在短期内，产量取决于固定资本与可变劳动的结合。总产量（TP）是总产出。平均产量（AP）是每单位劳动的产出：AP = TP ÷ L。边际产量（MP）是增加一名工人所增加的产出：MP = ΔTP ÷ ΔL。
Initially, specialisation and division of labour can make MP rise. Eventually, as more workers are added to fixed capital, MP falls because workers have less equipment to use. This pattern gives rise to the three stages of short-run production.
起初，专业化和劳动分工可能使边际产量上升。最终，随着更多工人加入固定资本，边际产量会下降，因为每个工人可使用的设备减少。这一规律形成了短期生产的三阶段。
5. The Law of Diminishing Marginal Returns | 边际收益递减规律
The law of diminishing marginal returns states that if a firm keeps increasing one variable input while holding other inputs fixed, the additional output from each extra unit of input will eventually fall. For example, adding more chefs to a fixed kitchen increases total meals, but after some point each extra chef adds fewer meals.
边际收益递减规律指出，如果企业在保持其他投入不变的情况下不断增加一种可变投入，每增加一单位投入所带来的额外产出最终会下降。例如，在厨房面积固定的情况下增加更多厨师，总餐数会增加，但超过某一点后，每增加一名厨师所增加的餐数会减少。
Diminishing returns is a short-run phenomenon. It is not the same as diseconomies of scale, which occur in the long run when all inputs are variable. Diminishing returns explains why marginal cost eventually rises as output expands.
边际收益递减是一个短期现象。它不同于规模不经济，后者发生在长期、所有投入都可变的情况下。边际收益递减解释了为什么随着产量扩大，边际成本最终上升。
6. Short-Run Cost Curves and Their Shapes | 短期成本曲线及其形状
The short-run average total cost curve is usually U-shaped. AFC falls continuously as output rises. AVC tends to fall at first due to specialisation, then rises because of diminishing returns. ATC falls while spreading fixed costs and rising efficiency dominate, then rises when diminishing returns push AVC up sharply.
短期平均总成本曲线通常呈 U 形。平均固定成本随产量上升持续下降。平均可变成本起初因专业化而下降，随后因边际收益递减而上升。当分摊固定成本和效率提高占主导时，平均总成本下降；当边际收益递减使平均可变成本急剧上升时，平均总成本上升。
MC intersects AVC and ATC from below at their minimum points. If MC is below ATC, producing more units lowers average cost; if MC is above ATC, producing more raises average cost.
边际成本曲线从下方分别穿过平均可变成本曲线和平均总成本曲线的最低点。如果边际成本低于平均总成本，增加产量会降低平均成本；如果边际成本高于平均总成本，增加产量会提高平均成本。
7. Revenue: Total, Average and Marginal Revenue | 收入：总收入、平均收入与边际收入
Total revenue (TR) is the money a firm receives from selling output: TR = Price × Quantity. Average revenue (AR) is revenue per unit: AR = TR ÷ Q = Price. Marginal revenue (MR) is the extra revenue from selling one more unit: MR = ΔTR ÷ ΔQ.
总收入（TR）是企业销售产出获得的货币：TR = 价格 × 数量。平均收入（AR）是每单位收入：AR = TR ÷ Q = 价格。边际收入（MR）是多销售一单位产品所增加的收入：MR = ΔTR ÷ ΔQ。
In perfect competition, firms are price takers, so AR = MR = Price, and the demand curve is horizontal. In imperfect competition, a firm must lower price to sell more, so MR lies below AR and can become negative.
在完全竞争中，企业是价格接受者，因此 AR = MR = 价格，需求曲线是水平的。在不完全竞争中，企业必须降低价格才能卖出更多产品，因此边际收入低于平均收入，甚至可能变为负值。
8. Profit: Accounting Profit, Normal Profit and Supernormal Profit | 利润：会计利润、正常利润与超常利润
Accounting profit is total revenue minus explicit costs such as wages, rent and materials. Economic profit subtracts both explicit costs and implicit opportunity costs, including the income the owner could have earned elsewhere.
会计利润是总收入减去显性成本，如工资、租金和原材料。经济利润则同时减去显性成本和隐性机会成本，包括企业主原本可以在其他地方获得的收入。
Normal profit is the minimum profit required to keep resources in their current use. It is treated as a cost in economics and occurs when total revenue equals total economic cost, so economic profit is zero. Supernormal profit, also called abnormal profit, is any profit above normal profit.
正常利润是维持资源用于当前用途所需的最低利润。在经济学中它被视为成本，当总收入等于总经济成本时出现，此时经济利润为零。超常利润又称超额利润，是超过正常利润的任何利润。
9. Profit Maximisation: MC = MR | 利润最大化：边际成本等于边际收入
A firm maximises profit where marginal cost equals marginal revenue, provided MC cuts MR from below. If MR > MC, producing one more unit adds more to revenue than to cost, so profit rises. If MC > MR, producing one more unit adds more to cost than to revenue, so profit falls.
企业在边际成本等于边际收入处实现利润最大化，且要求边际成本从下方穿过边际收入曲线。如果 MR > MC，多生产一单位带来的收入增加大于成本增加，利润上升；如果 MC > MR，多生产一单位带来的成本增加大于收入增加，利润下降。
Profit maximising output can be found by comparing MC and MR, not by simply maximising total revenue or minimising average cost. The firm may earn supernormal profit, normal profit or even losses depending on the level of average cost at that output.
利润最大化产量通过比较边际成本与边际收入确定，而不是简单地最大化总收入或最小化平均成本。根据该产量下平均成本的水平，企业可能获得超常利润、正常利润甚至亏损。
10. Long-Run Production and Returns to Scale | 长期生产与规模收益
In the long run, all factors of production are variable, so firms can change the scale of production. Returns to scale describe what happens to output when all inputs are increased by the same proportion.
在长期中，所有生产要素都是可变的，因此企业可以改变生产规模。规模收益描述的是当所有投入按相同比例增加时，产出会如何变化。
Increasing returns to scale occur when output rises by a larger proportion than inputs, often because of specialisation of labour and capital. Constant returns to scale occur when output rises by the same proportion. Decreasing returns to scale occur when output rises by a smaller proportion, often because coordination becomes harder.
规模收益
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