(x+y)/(y-x)=-(x+y)/(x-y)
移项
{（x+y）+（x+y）}/(x-y)=0
2(x+y)/(x-y)
分子分母同乘以（x+y)得
2（x+y）^2 /(x-y)(x+y)
化简得
(2x^2+4xy+y^2)/(x^2-y^2)