清华大学中级微观经济学讲义(清华 李稻葵)15.ppt

ChapterFifteen MarketDemand WhatIsthePurposeofThisChapter Weaggregateindividualdemandandformamarketdemandfunction Westudyapropertyofthemarketdemandfunction FromIndividualtoMarketDemandFunctions Thinkofaneconomycontainingnconsumers denotedbyi 1 n Consumeri sordinarydemandfunctionforcommodityjis FromIndividualtoMarketDemandFunctions Whenallconsumersareprice takers themarketdemandfunctionforcommodityjis FromIndividualtoMarketDemandFunctions p1 p1 p1 20 15 35 p1 p1 p1 p1 p1 p1 The horizontalsum ofthedemandcurvesofindividualsAandB AnImportantConcept Elasticity Elasticitymeasuresthe sensitivity ofonevariablewithrespecttoanother TheelasticityofvariableXwithrespecttovariableYis SlopeVs Elasticity Q Whynotjustusetheslopeofademandcurvetomeasurethesensitivityofquantitydemandedtoachangeinacommodity sownprice A Becausethevalueofsensitivitythendependsuponthe arbitrary unitsofmeasurementusedforquantitydemanded ArcandPointElasticities An average own priceelasticityofdemandforcommodityioveranintervalofvaluesforpiisanarc elasticity usuallycomputedbyamid pointformula Elasticitycomputedforasinglevalueofpiisapointelasticity PointOwn PriceElasticity pi Xi pi Whatistheown priceelasticityofdemandinaverysmallintervalofpricescenteredonpi istheelasticityatthepoint PointOwn PriceElasticity E g Supposepi a bXi ThenXi a pi band Therefore PointOwn PriceElasticity pi Xi a pi a bXi a b a 2 a 2b own priceelastic own priceinelastic own priceunitelastic PointOwn PriceElasticity E g Then so PointOwn PriceElasticity pi Xi everywherealongthedemandcurve RevenueandOwn PriceElasticityofDemand Sellers revenueis So RevenueandOwn PriceElasticityofDemand soif then andachangetopricedoesnotaltersellers revenue RevenueandOwn PriceElasticityofDemand butif then andapriceincreaseraisessellers revenue RevenueandOwn PriceElasticityofDemand Andif then andapriceincreasereducessellers revenue MarginalRevenueandOwn PriceElasticityofDemand Aseller smarginalrevenueistherateatwhichrevenuechangeswiththenumberofunitssoldbytheseller MarginalRevenueandOwn PriceElasticityofDemand p q denotestheseller sinversedemandfunction i e thepriceatwhichthesellercansellqunits Then so MarginalRevenueandOwn PriceElasticityofDemand and so MarginalRevenueandOwn PriceElasticityofDemand If then If then If then MarginalRevenueandOwn PriceElasticityofDemand Anexamplewithlinearinversedemand Then and MarginalRevenueandOwn PriceElasticityofDemand a a b p q a 2b MarginalRevenueandOwn PriceElasticityofDemand a a b p q a 2b q a b a 2b R q Summary TheKeyConcept Itisthepriceel