n = n!
For example, the number of ways of ordering the letters A, B, and C is 3!, or 6:ABC, ACB, BAC, BCA, CAB, and CBA.
These orderings are called the permutations of the letters A, B, and C.也可以用P 33表示.
Pkn = n!/ !
例如:1, 2, 3, 4, 5这5个数字构成不同的5位数的总数为5! = 120
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