已知x^2-xy-5y^2=0
所以x(x-y)=x^2-xy=5y^2
y/（x-y）-（x-y）/x
=[xy-(x-y)^2/[x(x-y)]
=(-x^2+xy-y^2)/(5y^2)
=(-5y^2-y^2)/(5y^2)
=-6/5