- $ A_n^k = \\\frac{{n!}}{{(n - k)!}}$.
- $ A_n^k = \\\frac{{k!}}{{(n - k)!}}$.
- $ A_n^k = \\\frac{{n!}}{{(k - n)!}}$.
- $ A_n^k = \\\frac{{n!}}{{k!(n - k)!}}$.