Answer :
To answer this question we need to remember that:
[tex]0!=1[/tex]and that a permutation is defined as:
[tex]_nP_k=\frac{n!}{(n-k)!}[/tex]Then, in this case we have:
[tex]_nP_n=\frac{n!}{(n-n)!}=\frac{n!}{0!}=\frac{n!}{1}=n![/tex]Therefore, we have that:
[tex]_nP_n=n![/tex]