a,b为实数,且ab=2,设P=a+1分之a+b+1分之b,Q=a+1分之1+b+1分之1,则P与Q的大小关系为( )

P=a/(a+1)+b/(b+1) Q=1/(a+1)+1/(b+1) P-Q =(a-1)/(a+1)+(b-1)/(b+1) =[(a-1)(b+1)+(b-1)(a+1)]/[(a+1)(b+1)] =(2ab-2)/(ab+a+b+1) =2(ab-1)/(3+a+b) 若a,b为实数，那么无法确定P,Q大小 选D 若a,b>0则P>Q选A