专题十七-排列组合

1. 乘法原理

1.1. For example, if there are 2 ways to get from A to B and three ways to get from B to C and two ways to get from C to D.

1.2. 2*3*2 is the ways of getting from A to D

2. 排列

2.1. p(n,r)=n!/(n-r)!

2.1.1. MATHC choose 3

2.1.2. p(5,3)=5!/(5-3)!

2.2. distinct objects can be permuted in n! permutations

3. 组合

3.1. (n r)=n!/r!(n-r)!

3.2. (n 0)=1

3.3. (n 1)=n

3.4. n n)=1

3.5. (n r)=(n n-r)

4. 重复字母排列问题

4.1. AABBC has 5!/2!/2! ways

5. 锁链法