y/(x+y)-x/(x-y)
问题描述：
y/(x+y)-x/(x-y)
答
y/(x+y)-x/(x-y)
=y(x-y) /(x+y)(x-y) -x(x+y) /(x+y) (x-y)
=[y(x-y) -x(x+y)] /(x+y) (x-y)
=[xy-y^2 -x^2-xy] /(x+y) (x-y)
=[-y^2-x^2] /(x+y) (x-y)
=-(x^2+y^2)/(x^2-y^2)