令 s=x+y,t=xy,
则 x(s-x)=t,x^2=sx-t,
同理,y^2=sy-t,
代入已知等式得
f(s,t)=s(x+y)-2t=s^2-2t,
用x、y替换s、t,可得
f(x,y)=x^2-2y.