令 x+y=t (1)y/x=s (2)则y=sx，代入（1），得x(1+s)=t，x=t/(1+s)从而 y=ts/(1+s)所以 f(t，s)=x² -y²=t²/(1+s)²-t²s²/(1+s)²=t²(1-s²)/(1+s)²=t²(1-s)/(1+s)从而f(x，y)=x²(1-y)/(1+y)
