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。
以上只是GMAT考题中经常涉及到的数学算术方面的问题,今后我们将陆续在新开辟的网上课堂中介绍代数、几何以及系统的习题、讲解,以帮助大家在GMAT数学考试中更好地发挥中国学生的优势,拿到让美国人瞠目结舌的成绩!
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