f(x-1)=x^2+1
令x-1=t,x=t+1
f(t)=(t+1)^2+1
=t^2+2t+2
f(x)=x^2+2x+2
f(x-1)=x^2+1
=(x-1)^2+2(x-1)+2
f(x)=x^2+2x+2