📚 Year 9 CAIE Economics: Formula & Theorem Quick Reference Handbook | 九年级CAIE经济：公式定理速查手册
This handbook provides a concise summary of the essential formulas, definitions, and theorems for Year 9 CAIE Economics. Use it as a quick reference to reinforce your understanding of fundamental economic concepts.
本手册简明总结了九年级 CAIE 经济课程中必须掌握的公式、定义和定理，可用作快速查阅工具，帮助你巩固基本经济概念的理解。
1. The Basic Economic Problem: Scarcity & Choice | 基本经济问题：稀缺性与选择
The fundamental economic problem is scarcity: resources are limited while human wants are unlimited. This forces every society to make choices about how to use its limited resources.
基本经济问题是稀缺性：资源是有限的，而人类的欲望是无限的。这迫使每个社会必须就如何使用有限资源做出选择。
Economists identify three key questions every economy must answer: What goods and services should be produced? How should they be produced? For whom should they be produced?
经济学家指出每个经济都必须回答三个关键问题：应该生产什么商品和服务？应该如何生产？为谁生产？
2. Opportunity Cost & Rational Decision-Making | 机会成本与理性决策
Opportunity cost is the value of the next best alternative that must be sacrificed when a choice is made. Every decision involves a trade-off.
机会成本指的是在做出选择时必须牺牲的次优替代方案的价值。每一项决策都涉及权衡取舍。
The concept can be summarised in a simple formula:
这一概念可以总结为一个简单公式：
Opportunity Cost = Value of best alternative given up
机会成本 = 放弃的次优方案的价值
Rational decision-makers weigh the marginal benefits against the marginal costs of a choice. They will undertake an action if the marginal benefit exceeds the marginal cost.
理性决策者会权衡选择的边际收益与边际成本。只要边际收益大于边际成本，他们就会采取该行动。
3. Production Possibility Frontier (PPF) | 生产可能性边界
A Production Possibility Frontier (PPF) shows the maximum combinations of two goods that an economy can produce with its existing resources and technology, assuming full and efficient use of resources.
生产可能性边界（PPF）展示了一个经济体在现有资源和技术条件下，且资源得到充分利用和有效利用时，所能生产的两种商品的最大组合。
Points on the curve are efficient; points inside the curve represent unemployment or inefficient use of resources; points outside are unattainable with current resources.
曲线上的点是有效率的；曲线内的点表示存在失业或资源利用无效率；曲线外的点用当前资源无法达到。
The slope of the PPF illustrates the opportunity cost of producing one more unit of a good. A bowed-out (concave) PPF indicates increasing opportunity cost.
PPF 的斜率说明了多生产一单位某商品的机会成本。向外弯曲的（凹向原点的）PPF 表明机会成本递增。
4. Demand: Law & Curve | 需求：定律与曲线
The law of demand states that, ceteris paribus, there is an inverse relationship between price and quantity demanded. As the price of a good rises, the quantity demanded falls; as price falls, the quantity demanded rises.
需求定律指出，在其他条件不变的情况下，价格与需求量之间存在反比关系。商品价格上升时，需求量下降；价格下降时，需求量上升。
A change in the good’s own price causes a movement along the demand curve (a change in quantity demanded). Any other factor that influences buyers will shift the entire demand curve.
商品自身价格的变动会导致沿着需求曲线的移动（需求量变动）。其他影响买家的因素则会使整条需求曲线发生位移。
Key demand shifters include: changes in income (normal vs inferior goods), tastes and preferences, prices of substitutes and complements, expectations of future prices, and the number of consumers in the market.
主要的需求移动因素包括：收入变化（正常商品与劣等商品）、口味与偏好、替代品和互补品的价格、对未来价格的预期，以及市场上消费者的数量。
5. Supply: Law & Curve | 供给：定律与曲线
The law of supply states that, ceteris paribus, there is a direct relationship between price and quantity supplied. As the price of a good rises, the quantity supplied rises; as price falls, the quantity supplied falls.
供给定律指出，在其他条件不变的情况下，价格与供给量之间存在正比关系。价格上升，供给量增加；价格下降，供给量减少。
A change in the good’s own price results in a movement along the supply curve. Shifts in the supply curve occur when other determinants change, such as costs of production, technology, taxes, subsidies, weather, and the number of sellers.
商品自身价格的变化导致沿着供给曲线的移动。当其他决定因素发生变化时，供给曲线将发生位移，例如生产成本、技术、税收、补贴、天气状况以及卖者数量。
6. Market Equilibrium &
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