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