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
组合:combination
A permutation can be thought of as a selection process in which objects are selected one by one in a certain order. If the order of selection is not relevant and only k objects are to be selected from a larger set of n objects, a different counting method is employed.
Specially consider a set of n objects from which a complete selection of k objects is to be made without regard to order, where 0n . Then the number of possible complete selections of k objects is called the number of combinations of n objects taken k at a time and is Ckn.
从n个元素中任选k个元素的数目为:
Ckn. = n!/ ! k!
例如:从5个不同元素中任选2个的组合为C25 = 5!/2! 3!= 10
排列组合的一些特性
加法原则:Rule of Addition
做某件事有x种方法,每种方法中又有各种不同的解决方法。例如第一种方法中有y1种方法,第二种方法有y2种方法,等等,第x种方法中又有yx种不同的方法,每一种均可完成这件事,即它们之间的关系用or表达,那么一般使用加法原则,即有:y1+ y2+。。。+ yx种方法。
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