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1、九十學年度經濟學競試乙組答案九十學年度經濟學競試甲組選擇題答案九十學年度經濟學競試乙組選擇題答案九十學年度經濟學競試乙組問答題答案本部分第一題第一小題原擬要求推導該國儲蓄函數，因繕打錯誤故無法求解。1.假設某國之消費與投資函數分別如下：(1)請推導該國儲蓄函數。(2)請用預擬支出等於所得的條件繪圖求取該國均衡產出。(1)儲蓄函數為：(2)預擬支出為：預擬支出等於所得的條件在圖形上即為預擬支出與45°線交點（如下圖）YAE45°線12502.（1）獨佔性競爭的短期均衡與獨佔者的短期均衡有何不同？（2）獨佔性競爭的短期均衡
2、與完全競爭者又有何不同？Answer:2-1a獨佔者的短期均衡Theprofit-maximizingoutputisq0,whereMR=MC;priceisp0,whichisaboveMCatthatoutput.TherulesforprofitmaximizationrequirethatMR=MCandp>AVC.(AVCisnotshowninthegraph,butitmustbebelowATC.)Whetherprofitsatq0arepositiveornegative(orzero)depen
3、dsonthepositionoftheATCcurve.Inpart(i),whereaveragetotalcostisATC1,therearepositiveprofits,asshownbytheshadedarea.(Orasinpart(ii),whereaveragetotalcostisATC2,profitsarezero.IfaveragetotalcostswereATC3,themonopolistwouldsufferthelossesshownbytheshadedarea.)(ii)Ze
4、ro(ornegative)profitsDollarsperUnitMCATC1p0q00MRDDollarsperUnitDMRMCq00p0(i)PositiveprofitShort-RunProfitMaximizationforaMonopolistATC2ATC32-1b獨佔性競爭的短期均衡Theshort-runpositionforamonopolisticallycompetitivefirmissimilartothatofamonopolist.Short-runprofitmaximizati
5、onoccursatES,theoutputforwhichMR=MC.PriceispSandquantityisqS.Profitsmayexist;inthisexampletheyareshownbytheshadedarea.Startingfromtheshort-runpositionshowninpart2-1b,entryofnewfirmsshiftsthefirm'sdemandcurvetotheleftandeliminatesprofits.qSpSESMCATC0DMROutputDoll
6、arsperUnitAtypicalfirmintheshortrun·ProfitMaximizationforaFirminMonopolisticCompetition2-2TheShort-RunProfit-MaximizingOutputChoiceofaCompetitiveFirmP=MR=ARMCAVCE0q0q*q1Output·DollarsperUnitThefirmchoosestheoutputforwhichp=MCabovethelevelofAVC.Whenp=MC,asatq*,th
7、efirmwoulddecreaseitsprofitsifitchangeditsoutput.Atanypointtotheleftofq*,suchasq0,priceisgreaterthanthemarginalcost,anditisworthwhileforthefirmtoincreaseoutput(asindicatedbythearrowontheleft).Atanypointtotherightofq*,suchasq1,priceislessthanthemarginalcost,andit
8、isworthwhileforthefirmtoreduceoutput(asindicatedbythearrowontheright).
