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