x+y-（xy+1）=x+y-xy-1=x(1-y)-(1-y)=(x-1)(1-y)因为|X|<1 |Y|<1所以（X-1）〈0 （1-Y）〉0则（x-1)(1-y)〈0既x+y〈xy+1
可能是xy+1＞x+y
