排列组合做错的常见原因有：不清楚定理公式、排列组合计算时可能情况讨论不清楚、粗心等等。
粗心原因非常好治，不存在对知识点的理解问题，多注意就可以做对。
而对前两种情况，要解决问题首先一定要清楚排列组合定理公式的原理和适用规则，以及考虑周到每一种可能出现的排列组合情况。
新航道的小编给大家分享的排列组合知识考点讲解文字版。
排列组合定义&公式
例题1：A certain restaurant offers 8 different salads, 5 different main courses, 6 different desserts. If customers choose one salad, one main course and two different desserts for their meal, how many different meals are possible?
A. 120
B. 240
C. 480
D. 600
E. 1200
答案：D
例题2：How many positive integers can be expressed as a product of two or more of the prime numbers 5,7,11 ,and 13 if no one product is to include the same prime factor more than once ?
A. Eight
B. nine
C. Ten
D. Eleven
E. Twelve
答案：D
相邻问题型——捆绑法
GRE · 排列组合
举例：A，B，C，D，E排列，A和B必须相邻
计算：
例题3：Six state governors meet at an annual convention. They line up in random order to pose for a photograph. If the governors of Alaska and Hawaii are among the six governors, how many different ways can the governors line up for the picture so that these two governors are adjacent?
A. 5
B. 10
C. 120
D. 240
E. 720
答案：D
不相邻问题型——插空法
举例：A，B，C，D，E排列，A和B不能相邻
计算：
例题4：In how many ways can Ann, Bob, Chuck, Don and Ed be seated in a row such that Ann and Bob are not seated next to each other?
A. 24
B. 48
C. 56
D. 72
E. 96
答案：D
思 考 题
1、The 5 letters in the list G, H, I, J, K are to be rearranged so that G is the 3rd letter in the list and H is not next to G. How many such rearrangements are ?
A. 60
B. 36
C . 24
D. 12
E.6
解题策略：本题难度为Medium。特殊情况特殊考虑且优先考虑
本题中G是特殊情况，因此我们先考虑G的位置：G在第三位，且H不能挨着G，所以H位置可能在第yi位/第五位，H放置位置有2种可能。H放置完成后，还剩3个位置随意安排，放置顺序分别有3种、2种、1种可能，因此可能的选座顺序有2*3*2*1=12种，答案选D。
2、The 20 people at a party are divided into n mutually exclusive groups in such a way that the number of people in any group does not exceed the number in any other group by more than 1.
Quantity A：The value of n if at least one of the groups consists of 3 people
Quantity B：6
A. Quantity A is greater.
B. Quantity B is greater.
C. The two quantities are equal.
D. The relationship cannot be determined from the information given.
解题策略：本题难度为Hard。本题中每一组的人数只能是整数，需要多次分情况讨论。
答案：D
① 2人与3人的组合：
2人*1组+3人*6组=20人，n=7>6 ；
或2人*4组+3人*4组=20人，n=8>6；
或2人*7组+3人*2组=20人，n=9>6
② 3人与4人的组合：
3人*4组+4人*2组=20人，n=6 ；n>=6 ，选D
如果你想了解更多GRE课程，留学规划或者有任何疑问，欢迎联系新航道重庆学校。
新航道重庆学校官网：cq.xhd.cn
新航道重庆学校电话：400-185-9090
推荐阅读：
一起来唠唠出国留学英国后的那些“坑”，用自己的血泪教训给大家排个雷~
关于QS排名前100的澳洲院校的花费大全来啦，想去这些学校的小伙伴们有没有准备好钱包呀？
