令x-y=U,y/x=V
得x=U/(1-V),y=UV/(1-V)
则f(x-y,y/x)=x^2-y^2=(x+y)(x-y)
=f(U,V)
=U^2(1+V)/(1-V)
即有f(x,y)=x^2(1+y)/(1-y)