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GMAT数学辅导：算术知识点全解析 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。 乘法原则：Rule of Multiplication 完成一件事有x个步骤，第一步有y1种方法，第二步有y2种方法，。。。，第x步有yx种方法，完成这件事一共有y1 y2。。。yx种方法。 | 
