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1、经济学原理第三版习题答案SOLUTIONSTOTEXTPROBLEMS:QuickQuizzes1.Whenacompetitivefirmdoublestheamountitsells,thepriceremainsthesame,soitstotalrevenuedoubles.2.Thepricefacedbyaprofit-maximizingfirmisequaltoitsmarginalcostbecauseifpricewereabovemarginalcost,thefirmcouldincreaseprofitsbyincreasingoutput,whileifpricew
2、erebelowmarginalcost,thefirmcouldincreaseprofitsbydecreasingoutput.Aprofit-maximizingfirmdecidestoshutdownintheshortrunwhenpriceislessthanaveragevariablecost.Inthelongrun,afirmwillexitamarketwhenpriceislessthanaveragetotalcost.3.Inthelongrun,withfreeentryandexit,thepriceinthemarketisequaltobothafirm
3、smarginalcostanditsaveragetotalcost,asFigure1shows.Thefirmchoosesitsquantitysothatmarginalcostequalsprice;doingsoensuresthatthefirmismaximizingitsprofit.Inthelongrun,entryintoandexitfromtheindustrydrivethepriceofthegoodtotheminimumpointontheaverage-total-costcurve.Figure1QuestionsforReview1.Acompeti
4、tivefirmisafirminamarketinwhich:(1)therearemanybuyersandmanysellersinthemarket;(2)thegoodsofferedbythevarioussellersarelargelythesame;and(3)usuallyfirmscanfreelyenterorexitthemarket.2.Figure2showsthecostcurvesforatypicalfirm.Foragivenprice(suchasP*),thelevelofoutputthatmaximizesprofitistheoutputwher
5、emarginalcostequalsprice(Q*),aslongaspriceisgreaterthanaveragevariablecostatthatpoint(intheshortrun),orgreaterthanaveragetotalcost(inthelongrun).Figure23.Afirmwillshutdowntemporarilyiftherevenueitwouldgetfromproducingislessthanthevariablecostsofproduction.Thisoccursifpriceislessthanaveragevariableco
6、st.4.Afirmwillexitamarketiftherevenueitwouldgetifitstayedinbusinessislessthanitstotalcost.Thisoccursifpriceislessthanaveragetotalcost.5.Afirmspriceequalsmarginalcostinboththeshortrunandthelongrun.Inboththeshortrunandthelongrun,priceequalsmarginalrevenue.Thefirmshouldincreaseoutputaslongasmarginalrev
7、enueexceedsmarginalcost,andreduceoutputifmarginalrevenueislessthanmarginalcost.Profitsaremaximizedwhenmarginalrevenueequalsmarginalcost.6.Thefirmspriceequalstheminimumofaveragetotalcostonlyinthelongrun.Intheshortrun,pricemaybegreaterthanaveragetotalcost,inwhichcasethefirmismakingprofits,orpricemaybe
8、lessthanaveragetotalcost,inwhichcasethefirmismakinglosses.Butthesituationisdifferentinthelongrun.Iffirmsaremakingprofits,otherfirmswillentertheindustry,whichwilllowerthepriceofthegood.Iffirmsaremakinglosses,theywillexittheindustry,whichwillraisethepriceofthegood.Entryorexitcontinuesuntilfirmsaremaki
9、ngneitherprofitsnorlosses.Atthatpoint,priceequalsaveragetotalcost.7.Marketsupplycurvesaretypicallymoreelasticinthelongrunthanintheshortrun.Inacompetitivemarket,sinceentryorexitoccursuntilpriceequalstheminimumofaveragetotalcost,thesupplycurveisperfectlyelasticinthelongrun.ProblemsandApplications1.Aco
10、mpetitivemarketisoneinwhich:(1)therearemanybuyersandmanysellersinthemarket;(2)thegoodsofferedbythevarioussellersarelargelythesame;and(3)usuallyfirmscanfreelyenterorexitthemarket.Ofthesegoods,bottledwaterisprobablytheclosesttoacompetitivemarket.Tapwaterisanaturalmonopolybecausetheresonlyoneseller.Col
11、aandbeerarenotperfectlycompetitivebecauseeverybrandisslightlydifferent.2.Sinceanewcustomerisofferingtopay$300foronedose,marginalrevenuebetween200and201dosesis$300.Sowemustfindoutifmarginalcostisgreaterthanorlessthan$300.Todothis,calculatetotalcostfor200dosesand201doses,andcalculatetheincreaseintotalcost.