The factorial is useful for counting the number of ways that a set of objects can be ordered. If a set of n objects is to be ordered from 1st to nth, there are n choices for the 1st object, n-1 choices for the 2nd object, n-2 choices for the 3rd object, and so on, until there is only 1 choice for the nth object. Thus, by the multiplication principle, the number of ways of ordering the n objects is

　　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

　　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:

【考试辅导：GMAT数学精解--算术概述(3)】相关文章：

★ GMAT资格考试数学辅导:GR数学总结（二）

★ GMAT考试1-GMAT数学12月总结(九)

★ GMAT数学基本概念知多少

★ GMAT考试辅导:GMAT的公理运用

★ GMAT考试1-GMAT数学12月总结(十)

★ GMAT考试1-GMAT数学12月总结(三)

★ GMAT数学精解--算术概述(3)

★ GMAT考试数学辅导11月数学总结(二)

★ GMAT考试1-GMAT数学12月总结(六)

★ 考试辅导:1/7GMAT数学试题