动直线a:x=t；直线b:y=x；直线c:y=-1/2x+2；则a,b交点D(t,t)；a,c交点E(t,-1/2t+2；DE中点(t,1/4t+1)则DE中垂线y=1/4t+1，与Y轴交点F(0,1/4t+1)则k(DF)=(3/4t-1)/t=(3t-4)/(4t)；k(EF)=(-3/4t+1)/t=-(3t-4)/(4t)显然k(DF)=-k(EF), k(DF)≠-1/k(EF)即⊿DPE中，∠DPE≠90°所以，只有∠PDE=90°或∠PED=90°令|-1/2t+2-t|=|t|==>|2-3/2t|=|t|==>t1=4/5,t2=4，∴P(0,4/5)或P(0,4)
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