# Math Help - proof

1. ## proof

How to prove that:

2. ## Re: proof

Originally Posted by Mhmh96
How to prove that:
Note that $\sum\limits_{k = 1}^5 {k!} = 153$ is a multiple of nine.

Also, if $N\ge 6$ then $6!={\color{blue}6}\cdot 5\cdot 4\cdot{\color{blue}3}\cdot 2={\color{blue}720}$ which is a multiple of nine.

If $N\ge 6$ then $N!$ is a multiple of nine. WHY?