(xy+1)-(x+y)
=xy-x-y+1
=x(y-1)-(y-1)
=(x-1)(y-1)
=(1-x)(1-y)
因为x<1,y<1
所以1-x>0,1-y>0
(1-x)(1-y)>0
所以(xy+1)-(x+y)>0
xy+1>x+y