SQA Economics Quick Reference: Formulas and Theorems | SQA 经济公式定理速查手册

📚 SQA Economics Quick Reference: Formulas and Theorems | SQA 经济公式定理速查手册

This handbook provides a quick reference for the essential formulas and theorems covered in the Year 11 SQA Economics syllabus. It brings together key microeconomic and macroeconomic relationships, allowing you to revise definitions, calculations, and theoretical links efficiently for exams and assessments.

本手册为 SQA 十一年级经济学课程中的核心公式与定理提供快速参考。整合了关键的微观与宏观经济关系，帮助你高效复习定义、计算与理论联系，以备考试和评估之需。

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1. Basic Economic Concepts and Opportunity Cost | 基本经济概念与机会成本

The fundamental economic problem is scarcity: resources are finite while human wants are infinite. Because of this, every choice involves an opportunity cost – the value of the next best alternative that is given up.

基本经济问题是稀缺性：资源有限而人类欲望无穷。因此，每一个选择都涉及机会成本——即所放弃的次优选择的价值。

Opportunity Cost of Good X = |ΔY / ΔX|

On a production possibility frontier (PPF), the opportunity cost of producing one more unit of good X is the amount of good Y sacrificed, measured by the absolute slope of the PPF.

在生产可能性边界(PPF)上，多生产一单位商品X的机会成本是放弃的商品Y的数量，即PPF斜率的绝对值。

A straight-line PPF indicates constant opportunity cost, while a concave (bowed-out) PPF reflects increasing opportunity cost as resources are not equally efficient in all uses.

直线PPF表示机会成本不变；而向外凸的PPF反映机会成本递增，因为资源在不同用途中的效率并不相同。

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2. Demand, Supply and Price | 需求、供给与价格

The law of demand states that, ceteris paribus, as the price of a good increases, the quantity demanded decreases. A linear demand function can be written as:

需求定律指出，在其他条件不变的情况下，商品价格上升时，需求量下降。线性需求函数可表示为：

Qd = a – bP

ice = 0) and b is the slope coefficient showing the response of quantity to price changes.

其中 a 为截距（价格为零时的需求量），b 为斜率系数，反映需求量对价格变动的反应程度。

The law of supply states that as price increases, quantity supplied increases, ceteris paribus. A linear supply function is:

供给定律指出，价格上升时供给量上升。线性供给函数为：

Qs = c + dP

where c is the intercept and d is the slope coefficient showing how responsive quantity supplied is to price changes.

其中 c 为截距，d 为斜率系数，反映供给量对价格变动的反应程度。

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3. 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 price. The basic formula is:

需求价格弹性衡量需求量对价格变动的反应程度。基本公式为：

PED = %ΔQd ÷ %ΔP

Using the midpoint (arc) method to avoid discrepancies when moving along the demand curve:

使用中点（弧）弹性法可避免沿需求曲线移动时的不一致：

PED = [ (Q₂ – Q₁) ÷ ((Q₁ + Q₂)/2) ] ÷ [ (P₂ – P₁) ÷ ((P₁ + P₂)/2) ]

The value of PED determines how total revenue (TR = P × Q) changes when price changes:

• If PED > 1 (elastic), a price fall increases TR.
• If PED < 1 (inelastic), a price rise increases TR.
• If PED = 1 (unit elastic), TR stays constant.

PED 的数值决定价格变动时总收益 (TR = P × Q) 的变化：PED > 1（富有弹性）时降价增加总收益；PED < 1（缺乏弹性）时提价增加总收益；PED = 1（单位弹性）时总收益不变。

Elasticity Value	Term	Impact on Total Revenue (TR)
PED > 1	Elastic	Price ↓ ⇒ TR ↑
PED < 1	Inelastic	Price ↑ ⇒ TR ↑
PED = 1	Unit Elastic	TR is maximised and unchanged

中文对照：弹性值 PED > 1 富有弹性，降价使总收入增加；PED < 1 缺乏弹性，提价使总收入增加；PED = 1 单位弹性，总收入最大化且不变。

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4. Income Elasticity and Cross-Price Elasticity | 收入弹性与交叉价格弹性

Income elasticity of demand (YED) shows how quantity demanded responds to a change in consumer income.

收入需求弹性 (YED) 显示需求量对消费者收入变动的反应。

YED = %ΔQd ÷ %ΔY

A positive YED indicates a normal good (YED > 1 for luxury, 0 < YED < 1 for necessity); a negative YED indicates an inferior good.

YED 为正表示正常品（奢侈品 YED > 1，必需品 0 < YED < 1）；YED 为负表示劣等品。

Cross-price elasticity of demand (XED) measures how the quantity demanded of one good (A) responds to a change in the price of another good (B).

交叉价格弹性 (XED) 衡量一种商品 (A) 的需求量对另一种商品 (B) 价格变动的反应。

XED = %ΔQd of A ÷ %ΔP of B

A positive XED means the goods are substitutes; a negative XED means they are complements. A value close to zero suggests the goods are unrelated.

XED 为正表示替代品；为负表示互补品；接近零则表示商品不相关。

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5. 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.

供给价格弹性衡量供给量对价格变动的反应程度。

PES = %ΔQs ÷ %ΔP

eriod supply is often perfectly inelastic (PES = 0), in the short run it is inelastic (0 <

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