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AP经济学重要概念：弹性

弹性是AP经济学考试考试的一个知识点，下面我用几道题目来解释一下这一章的知识要点。

1. Use the following diagram to answer the next question.

Refer to the diagram. Between the prices of $10 and $8, the price elasticity of demand is:

A) .5

B) .9

C) 1.11

D) 2

解释：我们要学会用弹性公式中的中位法(middle point method) 来计算弹性，分子为数量变化，分母为价格变化。

(22 – 18) / 20 ÷ (10 – 8) / 9 = .9.

2. Use the following diagram to answer the next question.

Refer to the diagram. Suppose total revenue at price P3 is the same as at price P2. Then, over the price range from P2 to P3, demand is:

A) relatively elastic

B) relatively inelastic

C) unit elastic

D) perfectly elastic

解释：如果总收入不变，需求变化和数量变化一定保持一致，这是单位弹性(unit elasticity)

3. Suppose that a 2% increase in income in the economy decreases the quantity of gadgets demanded by 1% at every possible price. This implies that:

A) the supply of gadgets is elastic

B) income elasticity is positive and gadgets are a normal good

C) income elasticity is negative and gadgets are a normal good

D) income elasticity is negative and gadgets are an inferior good

解释: 我们用需求的收入弹性公式计算income elasticity = –1/2 = –.5 如入增加，需求减少，商品是劣质品(inferior good)

4. The maker of a particular breakfast cereal found that increasing the price from $4.00 to $4.25 per box had no impact on total revenue, but increasing the price further to $4.50 reduced total revenue by 2%. Thus, the demand for the cereal is:

A) inelastic over the range $4.00 to $4.50

B) elastic over the range $4.00 to $4.25 but not over the range $4.25 to $4.50

C) unit elastic over the range $4.00 to $4.25 and elastic over the range $4.25 to $4.50

D) unit elastic over the range $4.00 to $4.25 and inelastic over the range $4.25 to $4.50

解释：价格变化，收入不变，是单位弹性(unit elastic)。进一步涨价，使得总收入下降2%， 说明涨价对需求的影响很大，这个阶段需求是富有弹性的(demand is elastic over this range)

5. A firm finds that its price elasticity of demand is 4.0. Currently, the firm is selling 2000 units per month at $5 per unit. If it wishes to increases its quantity sold by 10%, it must lower its price by:

A) $.40

B) $.50

C) 2.5%

D) 4.0%

解释：根据弹性公式：数量变化量除以价格变化量等于弹性。 10%/ X= 4 X=2.5%