(x/x-y)*(y^2/x+y)-(x^4y/x^4-y^4)/x^2/x^2+y^2
=x*y^2/(x^2-y^2)-(x^4y/(x^2-y^2)(x^2+y^2)) * (x^2+y^2)/x^2
=x*y^2/(x^2-y^2)-x^2y/(x^2-y^2)
=xy(y-x)/((x-y)(x+y))
= -xy/(x+y)