设n+2=x
所以(n+1)(n+2)(n+3)
=(x-1)*x*(x+1)
=(x^2-1)*x
=x^3-x
将n+2=x代入,得n^3+3n^2*2+3n*2^2+2^3-n-2
=n^3+6n^2+12n-n-2
=n^3+6n^2+11n-2