排列组合

　　排列数公式和组合数公式：

　　排列：

　　组合：

　　乘法原理：

　　一件事的完成可以分成n个步骤，完成其第一步有m1种方法，完成第二步有m2种方法……完成第n步有mn种方法，那么完成这件事一共有N=m1*m2*……*mn种不同的方法。

　　例题.If all of the room numbers in a certain hotel must be even numbers, and if each of the numbers uses all four of the digits 3,4,5 and 8, what is the greatest number of four-digit numbers that the hotel can have?

　　例题.What is the total number of different 4-digit integers in which the units’ digit is an odd integer while the thousands’ digit is an even integer?

　　例题.What is the total number of different 5-digit numbers that contain all of the digits 1,2,3,4, and 5 and

　　1) in which none of the odd digits occur next to each other?

　　2) in which none of the even digits occur next to each other?

　　平时，建议大家一定要注意GRE数学基本概念的加强理解，因为无论GRE数学考试怎样的题目都是从最基本的概念中演变而来的。