若f(x,y)=x^2-y^2,
则f(x+y,y/x)=(x+y)^2-(y/x)^2
若f(x+y,y/x)=x^2-y^2
令a=x+y ,b=y/x
则y=bx,a=x+bx,得：
x=a/(1+b),y=ab/(1+b)
故f(a,b)=a^2/(1+b)^2 -(ab)^2/(1+b)^2=a^2/(1+b)^2*(1-b^2)=a^2(1-b)/(1+b)
即f(x,y)=x^2(1-y)/(1+y)