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AP經濟學重要概念：彈性

彈性是AP經濟學考試考試的一個知識點，下面我用幾道題目來解釋一下這一章的知識要點。

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

解釋：我們要學會用彈性公式中的中位法(middle point method) 來計算彈性，分子為數量變化，分母為價格變化。

(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

解釋：如果總收入不變，需求變化和數量變化一定保持一致，這是單位彈性(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

解釋: 我們用需求的收入彈性公式計算income elasticity = –1/2 = –.5 如入增加，需求減少，商品是劣質品(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

解釋：價格變化，收入不變，是單位彈性(unit elastic)。進一步漲價，使得總收入下降2%， 說明漲價對需求的影響很大，這個階段需求是富有彈性的(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%

解釋：根據彈性公式：數量變化量除以價格變化量等于彈性。 10%/ X= 4 X=2.5%