武汉大学微观经济学教学用课件Chap_05

(Q2 Q1 )/[(Q 2 Q1 )/2] Price Elasticity of Demand = (P2 P1 )/[(P2 P1 )/2]
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Y
1 (Y1 Y2 ) Y X 1 X 2 2 e X X Y1 Y2 1 ( X1 X 2 ) 2
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The Midpoint Method: A Better Way to Calculate
需求量变动的百分比 需求价格弹性 价格变动的百分比
例子: 如果冰激凌蛋卷的价格从2美元上升到 2.2美元，你的购买量从10个下降到8个，你的 需求价格弹性计算如下：
(10 8) 100 20 ％ 10 2 (2.20 2.00) 100 10 ％ 2.00
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Elasticity and Its Application
弹性及其应用
Chapter 5
Elasticity . . . 弹性…
… is a measure of how much buyers and sellers respond to changes in market conditions. …是衡量买者和卖者对市场条件变动的 反应大小的指标。 u … allows us to analyze supply and demand with greater precision. …使我们可以更精确地分析供给与需求
(10 8 ) 100 20 percent 10 2 ( 2.20 2.00) 100 10 percent 2.00
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计算需求价格弹性
Percentage Change in Quantity Demanded Price Elasticity of Demand = Percentage Change in Price
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(10 8) 22 percent (10 8) / 2 2.32 (2.20 2.00) 9.5 percent (2.00 2.20) / 2
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Time Horizon 时间的长短
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Determinants of Price Elasticity of Demand
Demand tends to be more elastic :
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n n
中点法:一种计算弧弹性的更好方法 为了解决这一问题，人们选择自变量和因变量在变动范围内的均 值来作为变动的基点，这样，在同一区间内，不论自变量的变动 方向如何，所计算出的弹性系数都是相同的。弧弹性中点公式为 ：
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需求点弹性的计算公式
n
假定需求函数为Q=f（P），以dQ和dP分 别表示需求量和价格的无穷小的变动量 ，以ed表示需求弹性系数，则需求点弹性 的公式为：
中点法:一种计算百分比变化和弹性的更好方法
(Q2 Q1 )/[(Q 2 Q1 )/2] 需求价格弹性= (P2 P1 )/[(P2 P1 )/2]
例子:如果冰激凌蛋卷的价格从2美元上升到2.2
美元，你的购买量从10个下降到8个，利用中 点法你的需求价格弹性计算如下：
(10 8) 22 ％ (10 8) / 2 2.32 (2.20 2.00) 9.5 ％ (2.00 2.20) / 2
Example: If the price of an ice cream cone increases from $2.00 to $2.20 and the amount you buy falls from 10 to 8 cones the your elasticity of demand, using the midpoint formula, would be calculated as:
The Midpoint Method: A Better Way to Calculate Percentage Changes and Elasticities
(Q2 Q1 )/[(Q 2 Q1 )/2] Price Elasticity of Demand = (P2 P1 )/[(P2 P1 )/2]
Percentage Changes and Elasticities
The midpoint formula is preferable when calculating the price elasticity of demand because it gives the same answer regardless of the direction of the change.
弧弹性
n
上例种的弹性称为弧弹性（Arc Elasticity）它只是近似地测定函 数在某一区间平均弹性 问题：从A点到B点的弹性不同与B点到A点的弹性，例如
n
A点：价格＝2元，数量＝10
B点：价格＝2.2元，数量＝8 从A到B，价格上升了10％，数量减少了20％，需求的价格弹性为 20/10＝2 从B到A，价格下降了9％，数量增加了25％，需求的价格弹性为 25/9＝2.78 避免这个问题的一种方法是用中点法计算弹性
u
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Price Elasticity of Demand
u
Price elasticity of demand is the percentage change in quantity demanded given a percent change in the price. It is a measure of how much the quantity demanded of a good responds to a change in the price of that good.
计算需求价格弹性
用需求量变动的百分比除以价格变动的百 分比来计算需求价格弹性。
需求量变动的百分比 需求价格弹性= 价格变动的百分比
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Computing the Price Elasticity of Demand
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点弹性： 如果经济变量可以连续变动，则当X趋于零时，弹性公 式可表示为：
Y Y dY X e lim X 0 X dX Y X
弧弹性（Arc Elasticity）它只是近似地测定 函数在某一区间平均弹性；点弹性（Point Elasticity），它可以精确地测定函数在某 一点（X,Y）的弹性。
Price elasticity of demand Percentagechange in quatity demanded Percentagechange in price
Example: If the price of an ice cream cone increases from $2.00 to $2.20 and the amount you buy falls from 10 to 8 cones then your elasticity of demand would be calculated as:
u
u u
u
if the good is a luxury. the longer the time period. the larger the number of close substitutes. the more narrowly defined the market.
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中点法:一种计算百分比变化 和弹性的更好方法
中点法 是计算需求价格弹性的更好方法，因 为无论变动的方向如何，中点法给出了同一 个答案。
(Q2 Q1 )/[(Q 2 Q1 )/2] 需求价格弹性= (P2 P1 )/[(P2 P1 )/2]
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