（x/x+y+2y/x+y）乘以xy/x+2y除以（1/x+1/y）
=（(x+2y)/(x+y)）*xy/(x+2y)/（(x+y)/xy）
=（xy/(x+y)）*（xy/(x+y)）
=x^2y^2/(x+y)^2