(x^2 -y^2)/(xy) - (xy-y^2)/(xy-x^2)=(x^2-y^2)/(xy) - y(x-y)/[x(y-x)]=(x^2-y^2)/(xy) + y/x=(x^2-y^2)/(xy) + y^2/(xy)=x^2/(xy)=x/y
