题目应该还给个条件吧?x≠y
x^2-y^2=x^3-y^3
(x+y)(x-y)=(x-y)(x^2+xy+y^2)
(x-y)[(x+y)^2-xy-(x+y)]=0
x=y或(x+y)^2-xy-(x+y)=0
x≠y时
(x+y)^2-xy-(x+y)=0
xy＜(x+y)^2/4
所以xy=(x+y)^2-(x+y)0
所以 (3/4)(x+y)-1