## Algebra 1: Common Core (15th Edition)

We can prove this easily with math. We know that n = r. We know that: $_nC_r =\frac{_nP_r}{r!} = \frac{n!}{(n-r)!r!}$ So it follows: $_nC_n =\frac{_nP_n}{n!} = \frac{n!}{(n-n)!n!}$ $\frac{n!}{0!n!}$ Since 0! = 1: $\frac{n!}{n!}$ = 1