f(x+y,y/x)=x²-y²令x+y=u, y/x=vx=u/(1+v) y=uv/(1+v)f(u,v)=u²/(1+v)²-u²v²/(1+v)²=[u²/(1+v)²]·(1-v²)=u²(1-v)/(1+v)即f(x,y)=x²(1-y)/(1+y)
