📚 Year 9 CIE Economics Formula & Theorem Quick Reference Handbook | 九年级CIE经济学公式定理速查手册
This handbook compiles the essential definitions, laws, and formulas prescribed for the Year 9 CIE Economics syllabus. Use it as a rapid revision tool to master the quantitative and qualitative fundamentals of microeconomics, including opportunity cost, demand, supply, elasticity, and revenue calculations.
本手册汇编了九年级CIE经济教学大纲要求的基本定义、定律与公式。你可以把它当作快速复习工具，掌握微观经济学的定量与定性基础，包括机会成本、需求、供给、弹性以及收入计算等内容。
1. Scarcity, Choice and Opportunity Cost | 稀缺性、选择与机会成本
Scarcity is the fundamental economic problem: finite resources cannot satisfy all human wants. As a result, economic agents must make choices, and every choice involves an opportunity cost.
稀缺性是基本的经济问题：有限的资源无法满足所有人类欲望。因此，经济主体必须做出选择，而每一次选择都涉及机会成本。
Opportunity cost is defined as the value of the next best alternative forgone when a decision is taken. It is not the sum of all alternatives, but only the single most valuable option sacrificed.
机会成本的定义是做出决策时所放弃的次优选择的价值。它不是所有备选方案的总和，而仅指被牺牲的那个最有价值的单一选项。
Opportunity Cost = Value of Next Best Alternative Forgone
For example, if a government spends $10 million on a new motorway instead of building a hospital, the opportunity cost is the value of the forgone hospital, not the money itself.
例如，如果政府花费1000万美元修建一条新高速公路，而不是建设一家医院，那么机会成本就是所放弃医院的价值，而不是那笔钱本身。
2. Factors of Production | 生产要素
Resources are categorised into four factors of production: land, labour, capital, and enterprise. Each factor earns a corresponding income: rent, wages, interest, and profit respectively.
资源分为四类生产要素：土地、劳动、资本和企业才能。每种要素分别获得相应的收入：地租、工资、利息和利润。
The quality and quantity of these factors determine a country's productive capacity. Improvements in education or technology shift the Production Possibility Curve outward.
这些要素的质量和数量决定了一个国家的生产能力。教育或技术的进步会使生产可能性曲线向外移动。
The law of diminishing returns states that as more units of a variable factor are added to a fixed factor, the marginal output will eventually decline.
边际报酬递减规律表明，不断增加可变要素的投入至固定要素之上，边际产出最终会下降。
3. Production Possibility Curve (PPC) | 生产可能性曲线
A PPC shows the maximum combinations of two goods an economy can produce when all resources are fully and efficiently employed. Points on the curve are efficient; points inside indicate underemployment.
生产可能性曲线表示一个经济体在所有资源得到充分和有效利用时所能生产的两种商品的最大组合。曲线上的点代表有效率，曲线内部的点则表示资源未充分利用。
The slope of the PPC represents the marginal rate of transformation (MRT), which is the opportunity cost of producing one more unit of Good X in terms of Good Y sacrificed.
PPC的斜率表示边际转换率，即多生产一单位商品X所必须牺牲的商品Y的数量，也就是机会成本。
MRT = ΔY / ΔX
A concave PPC reflects increasing opportunity cost, meaning that as more of one good is produced, the economy gives up increasingly larger amounts of the other good.
凹向原点的PPC反映了机会成本递增，意味着随着一种商品产量的增加，经济体需要牺牲的另一种商品的数量越来越多。
4. Demand | 需求
The law of demand states that, ceteris paribus, as the price of a good rises, the quantity demanded falls, creating a downward-sloping demand curve.
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，从而形成一条向下倾斜的需求曲线。
Changes in price cause movements along the demand curve (contraction or extension). Shifts of the entire demand curve are caused by non-price determinants such as income, tastes, prices of substitutes or complements, and expectations.
价格变化引起沿需求曲线的移动（收缩或扩张）。整条需求曲线的平移则由非价格决定因素造成，例如收入、偏好、替代品或互补品的价格以及预期。
An extension in demand means quantity demanded rises due to a fall in price; a contraction means quantity demanded falls due to a rise in price.
需求量的扩张是指价格下降导致的需求量增加；需求量的收缩是指价格上升导致的需求量减少。
5. Supply | 供给
The law of supply states that, ceteris paribus, as the price of a good rises, the quantity supplied increases, resulting in an upward-sloping supply curve.
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，从而形成一条向上倾斜的供给曲线。
Movement along the supply curve occurs when price changes. Shifts in the supply curve result from changes in costs of production, technology, taxes, subsidies, or the number of firms in the market.
当价格变化时，供给量沿供给曲线移动。供给曲线的平移则由生产成本、技术、税收、补贴或市场中企业数量的变化引起。
A rightward shift of supply represents an increase in supply at every price level, often due to improved technology or lower input prices.
供给曲线向右平移表示在每个价格水平下供给量都有所增加，这通常由技术进步或投入品价格下降所致。
6. Market Equilibrium | 市场均衡
Market equilibrium occurs at the price where quantity demanded equals quantity supplied. At this point, there is no tendency for price to change, and the market clears.
市场均衡发生在需求量等于供给量的价格水平上。此时，价格没有内在变动趋势，市场出清。
Qd = Qs at Equilibrium Price P*
Excess demand (shortage) exists when price is below equilibrium: Qd > Qs, putting upward pressure on price. Excess supply (surplus) exists when price is above equilibrium: Qs > Qd, causing downward pressure.
当价格低于均衡水平时出现超额需求（短缺）：Qd > Qs，给价格带来上升压力。当价格高于均衡水平时出现超额供给（过剩）：Qs > Qd，造成价格下降压力。
The equilibrium price and quantity can change when either demand or supply shifts. A rise in demand lifts both equilibrium price and quantity; a rise in supply lowers price but increases quantity.
需求或供给任一曲线平移时，均衡价格与数量都会发生变化。需求增加会同时提高均衡价格和数量；供给增加会降低价格但增加数量。
7. Price Elasticity of Demand (PED) | 需求价格弹性
Price elasticity of demand measures the responsiveness of quantity demanded to a change in the good's own price. It is always a negative number, but economists usually ignore the minus sign.
需求价格弹性衡量需求量对商品自身价格变化的反应程度。弹性值总是负数，但经济学家通常忽略负号。
PED = %ΔQd / %ΔP
elastic demand is horizontal (PED = ∞); perfectly inelastic demand is vertical (PED = 0).
当PED > 1时，需求富有弹性；PED < 1时缺乏弹性；PED = 1时为单位弹性。完全弹性需求呈水平线（PED = ∞），完全无弹性需求呈垂直线（PED = 0）。
Determinants of PED include the number and closeness of substitutes, the degree of necessity, the proportion of income spent, and the time period considered.
决定需求价格弹性的因素包括替代品的数量和相似程度、商品的必需程度、支出占收入的比例以及考察的时间长短。
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. It is usually positive because higher prices incentivise firms to produce more.
供给价格弹性衡量供给量对价格变化的反应程度。它通常为正，因为更高的价格激励企业增加产出。
PES = %ΔQs / %ΔP
determinant is the flexibility of the production process and the time available to adjust.
当PES > 1时供给富有弹性，PES < 1时缺乏弹性，PES = 1时为单位弹性。关键决定因素在于生产过程的灵活性以及可用于调整的时间。
In the short run, supply tends to be inelastic because firms cannot easily change all inputs. Over the longer run, supply becomes more elastic as all factors become variable.
在短期，供给往往缺乏弹性，因为企业难以轻易改变所有投入。在长期，随着所有要素均可变动，供给变得更具弹性。
9. Consumer and Producer Surplus | 消费者剩余与生产者剩余
Consumer surplus is the difference between the maximum price consumers are willing to pay and the market price they actually pay. It measures consumer welfare.
消费者剩余是消费者愿意支付的最高价格与实际支付的市场价格之间的差额，用来衡量消费者福利。
Consumer Surplus = Willingness to Pay - Market Price
Producer surplus is the difference between the market price received by producers and the minimum price at which they are willing to supply. It measures producer welfare.
生产者剩余是生产者获得的市场价格与其愿意接受的最低供给价格之间的差额，用以衡量生产者福利。
Producer Surplus = Market Price - Minimum Supply Price
Total economic welfare is the sum of consumer and producer surplus. Market equilibrium maximises this total surplus, assuming no externalities.
总经济福利是消费者剩余与生产者剩余之和。在没有外部性的假设下，市场均衡使总剩余最大化。
10. Taxation and Subsidies | 税收与补贴
An indirect tax shifts the supply curve vertically upwards by the amount of the tax. The new equilibrium results in a higher price for consumers and a lower price kept by producers.
间接税使供给曲线按税收金额垂直上移。新均衡下，消费者支付的价格更高，生产者保留的价格更低。
The tax burden is distributed between consumers and producers depending on the relative elasticities of demand and supply. The more inelastic side of the market bears a larger share.
税收负担根据需求与供给的相对弹性在消费者与生产者之间分配。市场中更缺乏弹性的一方将承担较大份额。
Tax per unit = Price paid by consumers - Price received by producers
A subsidy lowers production costs and shifts the supply curve downwards. It reduces the market price for consumers and raises the effective price received by producers.
补贴降低了生产成本，使供给曲线下移。它降低了消费者支付的市场价格，并提高了生产者实际获得的价格。
11. Revenue, Costs and Profit | 收入、成本与利润
Firms use simple formulas to analyse their financial performance. Total revenue is the income generated from selling a product.
企业使用简单的公式来分析财务表现。总收益是从产品销售中获得的收入。
Total Revenue (TR) = Price (P) × Quantity Sold (Q)
Average revenue is revenue per unit, which equals the price when all units are sold at the same price.
平均收益是每单位收益，当所有单位以相同价格出售时，平均收益等于价格。
Average Revenue (AR) = TR / Q = P
Marginal revenue is the additional revenue from selling one more unit. It is crucial for profit-maximisation decisions.
边际收益是额外出售一单位产品所带来的新增收益，对利润最大化决策至关重要。
Marginal Revenue (MR) = ΔTR / ΔQ
Profit is the surplus remaining after all costs are deducted from total revenue.
利润是总收益扣除所有成本之后的剩余部分。
Profit = TR - Total Cost (TC)
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