f(x-1)=x^2-2x-3
令 x-1=t
x=t+1
则f(t)=(t+1)^2-2(t+1)-3
=(t+1)(t+1-2)-3
=(t+1)(t-1)-3
=t^2-1-3
=t^2-4
令t=x
则f(x)=x^2-4