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1、Chapter FifteenMarket Demand市场需求市场需求StructureuFrom Individual to Market Demand FunctionsuElasticitiesuRevenue and own-price elasticity of demanduMarginal revenue and price elasticityFrom Individual to Market Demand FunctionsuThink of an economy containing n consumers,denoted by i=1,n.uConsumer is or
2、dinary demand function for commodity j isFrom Individual to Market Demand FunctionsuWhen all consumers are price-takers,the market demand function for commodity j isuIf all consumers are identical then where M=nm.From Individual to Market Demand FunctionsuThe market demand curve is the“horizontal su
3、m”of the individual consumers demand curves.uE.g.suppose there are only two consumers;i=A,B.From Individual to Market Demand Functionsp1p12015p1p1”p1p1”From Individual to Market Demand Functionsp1p1p12015p1p1”p1p1”p1From Individual to Market Demand Functionsp1p1p12015p1p1”p1p1”p1p1”From Individual t
4、o Market Demand Functionsp1p1p1201535p1p1”p1p1”p1p1”The“horizontal sum”of the demand curvesof individuals A and B.Elasticities（弹性）uElasticity measures the“sensitivity”of one variable with respect to another.uThe elasticity of variable X with respect to variable Y isEconomic Applications of Elasticit
5、yuEconomists use elasticities to measure the sensitivity oflquantity demanded of commodity i with respect to the price of commodity i(own-price elasticity of demand，需求的自价格弹性需求的自价格弹性)ldemand for commodity i with respect to the price of commodity j(cross-price elasticity of demand，需求的交叉价格弹性需求的交叉价格弹性).
6、Economic Applications of Elasticityldemand for commodity i with respect to income(income elasticity of demand 需求的收入弹需求的收入弹性性)lquantity supplied of commodity i with respect to the price of commodity i(own-price elasticity of supply 供给的自价格弹性供给的自价格弹性)Economic Applications of Elasticitylquantity supplie
7、d of commodity i with respect to the wage rate(elasticity of supply with respect to the price of labor)land many,many others.Own-Price Elasticity of DemanduQ:Why not use a demand curves slope to measure the sensitivity of quantity demanded to a change in a commoditys own price?Own-Price Elasticity o
8、f DemandX1*5501010slope=-2slope=-0.2p1p1In which case is the quantity demandedX1*more sensitive to changes to p1?X1*Own-Price Elasticity of Demand5501010slope=-2slope=-0.2p1p1X1*X1*In which case is the quantity demandedX1*more sensitive to changes to p1?Own-Price Elasticity of Demand5501010slope=-2s
9、lope=-0.2p1p110-packsSingle UnitsX1*X1*In which case is the quantity demandedX1*more sensitive to changes to p1?Own-Price Elasticity of Demand5501010slope=-2slope=-0.2p1p110-packsSingle UnitsX1*X1*In which case is the quantity demandedX1*more sensitive to changes to p1?It is the same in both cases.O
10、wn-Price Elasticity of DemanduQ:Why not just use the slope of a demand curve to measure the sensitivity of quantity demanded to a change in a commoditys own price?uA:Because the value of sensitivity then depends upon the(arbitrary)units of measurement used for quantity demanded.Own-Price Elasticity
11、of Demandis a ratio of percentages and so has nounits of measurement.Hence own-price elasticity of demand is a sensitivity measure that is independent of units of measurement.Point Own-Price ElasticityE.g.Suppose pi=a-bXi.Then Xi=(a-pi)/b andTherefore,Point Own-Price ElasticitypiXi*pi=a-bXi*aa/bPoin
12、t Own-Price ElasticitypiXi*pi=a-bXi*aa/bPoint Own-Price ElasticitypiXi*pi=a-bXi*aa/bPoint Own-Price ElasticitypiXi*pi=a-bXi*aa/bPoint Own-Price ElasticitypiXi*api=a-bXi*a/bPoint Own-Price ElasticitypiXi*api=a-bXi*a/ba/2a/2bPoint Own-Price ElasticitypiXi*api=a-bXi*a/ba/2a/2bPoint Own-Price Elasticity
13、piXi*api=a-bXi*a/ba/2a/2bPoint Own-Price ElasticitypiXi*api=a-bXi*a/ba/2a/2bown-price elastic（有弹性）有弹性）own-price inelastic（缺乏弹性）缺乏弹性）Point Own-Price ElasticitypiXi*api=a-bXi*a/ba/2a/2bown-price elasticown-price inelastic(own-price unit elastic)单位弹性单位弹性Point Own-Price ElasticityE.g.ThensoPoint Own-Pri
14、ce ElasticitypiXi*everywhere alongthe demand curve.Revenue and Own-Price Elasticity of DemanduIf raising a commoditys price causes little decrease in quantity demanded,then sellers revenues rise.lHence own-price inelastic demand causes sellers revenues to rise as price rises.uIf raising a commoditys
15、 price causes a large decrease in quantity demanded,then sellers revenues fall.lHence own-price elastic demand causes sellers revenues to fall as price rises.Revenue and Own-Price Elasticity of DemandSellers revenue isRevenue and Own-Price Elasticity of DemandSellers revenue isSoRevenue and Own-Pric
16、e Elasticity of DemandSellers revenue isSoRevenue and Own-Price Elasticity of DemandSellers revenue isSoRevenue and Own-Price Elasticity of DemandRevenue and Own-Price Elasticity of Demandso ifthenand a change to price does not altersellers revenue.Revenue and Own-Price Elasticity of Demandbut ifthe
17、nand a price increase raises sellersrevenue.Revenue and Own-Price Elasticity of DemandAnd ifthenand a price increase reduces sellersrevenue.Revenue and Own-Price Elasticity of DemandIn summary:Own-price inelastic demand;price rise causes rise in sellers revenue.Own-price unit elastic demand;price ri
18、se causes no change in sellersrevenue.Own-price elastic demand;price rise causes fall in sellers revenue.Marginal Revenue and Own-Price Elasticity of DemanduA sellers marginal revenue is the rate at which revenue changes with the number of units sold by the seller.Marginal Revenue and Own-Price Elas
19、ticity of Demandp(q)denotes the sellers inverse demand function;i.e.the price at which the seller can sell q units.ThensoMarginal Revenue and Own-Price Elasticity of DemandandsoMarginal Revenue and Own-Price Elasticity of Demandsays that the rateat which a sellers revenue changeswith the number of u
20、nits it sellsdepends on the sensitivity of quantitydemanded to price;i.e.,upon theof the own-price elasticity of demand.Marginal Revenue and Own-Price Elasticity of DemandIfthenIfthenIfthen Selling onemore unit raises the sellers revenue.Selling onemore unit reduces the sellers revenue.Selling onemo
21、re unit does not change the sellersrevenue.Marginal Revenue and Own-Price Elasticity of DemandIfthenIfthenIfthenMarginal Revenue and Own-Price Elasticity of DemandAn example with linear inverse demand.ThenandMarginal Revenue and Own-Price Elasticity of Demandaa/bpqa/2bMarginal Revenue and Own-Price Elasticity of Demandaa/bpqa/2bq$a/ba/2bR(q)Income ElasticityuNormal good:0uInferior good:1uNecessary good:0 1A Special PropertyuWeighted average of income elasticities of all goods is one with the weights being expenditure shares of the goods.uIn the case of 2 goods,