设u=x+y, v=y/x
可以解出x=u/(v+1),y=uv/(v+1)
所以f(u,v)=[u/(v+1)]^2-[uv/(v+1)]^2=(1-v^2)u^2/(v+1)^2=(1-v)u^2/(1+v)
所以f(x,y)=(1-y)x^2/(1+y)
设u=x+y, v=y/x
可以解出x=u/(v+1),y=uv/(v+1)
所以f(u,v)=[u/(v+1)]^2-[uv/(v+1)]^2=(1-v^2)u^2/(v+1)^2=(1-v)u^2/(1+v)
所以f(x,y)=(1-y)x^2/(1+y)