设f(x)=kx+b
则f[f(x)]=f(kx+b)=k(kx+b)+b
=k²x+kb+b
对应项系数相等
有k²=4,kb+b=-1
则k=-2,b=1或k=2,b=-1/3
则f(x)=-2x+1或2x-1/3