AP经济学重要概念--- 弹性

 

弹性是AP经济学考试必考的一个知识点，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%

 

6. The cross elasticity of demand between two goods is reported to be +0.2. This implies that:

  A) a 2% increase in the price of one shifts the demand curve for the other to the left by 1%

  B) the two goods are complements

  C) the two goods are substitutes

  D) both goods are normal goods

解释：根据交叉弹性为正，说明两种商品是替代品。

 

7. Which of the following is likely to have the most elastic demand?

  A) Food

  B) Fruit

  C) Bananas

  D) Dole brand bananas

解释：对商品的定义越窄（narrow definition），商品的需求价格弹性越大。这里某个品牌的香蕉（Dole brand bananas）是四个选项中对商品定义最窄的。

 

8. While it is often relatively easy to shift land from production of one type of grain to another, the process takes a considerable amount of time. This implies that:

  A) a change in the demand for wheat will not affect its price in the short run

  B) the long run supply of oats is more elastic than the long run supply of wheat

  C) a change in the demand for corn will change quantity supplied more in the short run than the long run

  D) the supply of barley is more elastic in the long run than the short run

解释：短期内资源难以从一种转移到另外一种，但是随着时间的推移，可以通过增加或减少耕种的土地来应对价格变化，因此长期富有弹性。

 

9. Suppose legalization—and subsequent regulation—of heroin and cocaine reduces their prices by 50%. Estimates suggest the total quantity of heroin and cocaine demanded would rise by 83% and 42%, respectively. Consequently, legalization would:

  A) increase total expenditures on both heroine and cocaine

  B) decrease total expenditures on both heroine and cocaine

  C) increase total expenditures on heroine and decrease total expenditures on cocaine

  D) decrease total expenditures on heroine and increase total expenditures on cocaine

解释：对heroin的需求是elastic (83% > 50%) ，对cocaine的需求是 inelastic (42% < 50%). 价格下降增加对heroin的消费, 价格下降减少对 cocaine的需求

 

10.Assume that the price of product Y decreases by 5% and the quantity supplied decreases by 2%. The coefficient of price elasticity of supply for good Y is:

  A) negative and therefore Y is an inferior good

  B) less than one and therefore supply is inelastic

  C) more than one and therefore supply is elastic

  D) negative and therefore the supply curve is downsloping

解释：

供给的价格弹性是2/5, 因此供给价格弹性为inelastic.

 

以上是弹性章节中的典型例题，大家在准备AP考试中要注意掌握需求的价格弹性，收入的价格弹性，需求的交叉弹性和供给的价格弹性，大家要对各种弹性的定义、公式、图像全面理解，才能完全掌握这部分知识。