f(x0=kx+b
f[f(x0]=k(kx+b)+b
=k²x+b(k+1)=2x-1
k²=2
b(k+1)=-1
k=±√2,b=1/(k+1)
所以f(x)=-√2x-1-√2,f(x)=√2x+√2-1