设√x+1=k (k>=1)
x=(k-1)^2
f(k)=(k-1)^2+2√(k-1)^2
=k^2-2k+1+2k-2
=k^2-1 (k>=1)
f(x)=x^2-1 (x>=1)
f(x-1)=(x-1)^2-1=x^2-2x