x²-3x+2
=x²-x-2x+2
=x(x-1)-2(x-1)
=(x-2)(x-1)
=(x-1-1)(x-1)
则f(x)=x(x-1)
则f(x+1)=(x+1)(x+1-1)
=x(x+1)
=x²+x