一张图串起六个单元
从「稀缺」到「需求价格弹性」

Scarcity稀缺性

Wants are unlimited, resources are limited. 欲望无限，资源有限。

Choice选择

Because resources are scarce, we cannot have everything — so we must choose. 因为稀缺，所以必须取舍。

Opportunity cost机会成本

The value of the next best alternative forgone. 被放弃的「次优选择」的价值。

Three basic questions三个基本问题

What to produce? How to produce? For whom to produce? 生产什么、怎么生产、为谁生产。

Economic vs free goods经济品 / 免费品

Economic good: scarce, has an opportunity cost. Free good: not scarce, zero opportunity cost. 判断标准是「稀不稀缺」，不是「要不要钱」。

Factors of production生产要素 + 报酬

Land → rent · Labour → wages · Capital → interest · Enterprise → profit. 土地／劳动／资本／企业家才能。

Ceteris paribus其他条件不变

"All other things being equal." Change one variable, freeze the rest. 只让一个变量动，其余按住。

Rational consumers Maximise the benefits 理性消费者

— cheapest price 便宜的价格 — best quality 好的质量

Rational producers Profit-maximising firms利润最大化企业

— cheap raw materials 便宜的材料 — higher price/revenue 高的售价

Why consumers not rational消费者为什么不理性

difficult to calculate the benefits, buying habits, influenced by others

Why producers not rational生产商为什么不理性

influenced by the behaviour of other people in the organisation, objectives, not-for-profit organisation

Law of demand需求定律

As price rises, quantity demanded falls (ceteris paribus). 价格上升，需求量下降。

The demand curve需求曲线

Downward sloping. Price on the vertical axis, quantity on the horizontal axis. 纵轴价格、横轴数量，向右下倾斜。

Movement along沿曲线移动

Caused only by a change in the good's own price. 只有自身价格变。

Shift of the curve曲线移动

Caused by any determinant other than price. 价格以外的因素变。

Factors for shifting位移的因素

Law of supply供给定律

As price rises, quantity supplied rises (ceteris paribus). 价格上升，供给量上升。

The supply curve供给曲线

Upward sloping. 向右上倾斜。

Movement vs shift沿曲线 vs 曲线移动

Own price change → movement. Anything else → shift. 逻辑与需求完全一致。

Cost of production生产成本

Wages, raw materials, energy, rent. 成本上升 → 供给减少（左移）。

Technology技术

Better technology lowers unit cost → supply rises (shift right). 技术进步 → 右移。

Taxes & subsidies税收与补贴

Tax raises cost → supply shifts left. Subsidy lowers cost → shifts right. 考点：方向别写反。

Natural factors自然因素

Weather, natural disasters (agriculture) 天气

Equilibrium均衡

Where quantity demanded = quantity supplied. The price has no tendency to change. 供需相等，价格没有变动趋势。

Excess supply过剩 surplus

Price above equilibrium → Qs > Qd → price falls. 价格过高，卖方积压，价格下行。

Excess demand短缺 shortage

Price below equilibrium → Qd > Qs → price rises. 价格过低，排队抢购，价格上行。

Price mechanism价格机制三大功能

Rationing 配给 · Signalling 信号 · Incentive 激励. 价格用三件事解决稀缺。

Both curves shift两条曲线同时移动

One of P or Q becomes uncertain — you must say which and why. 必有一项无法确定，要主动说明。

Formula公式

PED = %ΔQd ÷ %ΔP 需求量变化的百分比 ÷ 价格变化的百分比。

Calculation算法示例

Price 10 → 12 (+20%). Quantity 100 → 80 (−20%).
PED = 20 ÷ 20 = 1 (unit elastic).

Five ranges五档弹性

0 perfectly inelastic · <1 inelastic · =1 unit elastic · >1 elastic · ∞ perfectly elastic. 数值越大越敏感。

Determinants决定因素

Number of substitutes · Necessity or luxury · Proportion of income · Time period · Habit / addiction. 替代品越多、越是奢侈品、占收入越大、时间越长 → 越弹性。

PED & total revenue弹性与总收入

Elastic: cut price → TR rises. Inelastic: raise price → TR rises. Unit elastic: TR is maximised. 这是定价决策的全部逻辑。

Who cares?谁在用 PED

Firms set prices; governments choose which goods to tax. 需求越不弹性，税越容易转嫁给消费者。