排列數
將組合作排序就是排列
$\small\left( \begin{array}{} 3 \\ 2 \end{array} \right) $×2!
| 1 | a | b | |
| 2 | b | a | |
| 3 | c | d | |
| 4 | d | c | |
| 5 | e | f | |
| 6 | f | e |
$\small\left( \begin{array}{} n \\ m \end{array} \right) $ × m! = $\large\frac{n!}{m!(n-m)!}$× m!= $\large\frac{n!}{(n-m)!}$
$P_m^n$=$\large\frac{n!}{(n-m)!}$
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