专题十七-排列组合
von Rey Skywalker
1. 组合
1.1. (n r)=n!/r!(n-r)!
1.2. (n 0)=1
1.3. (n 1)=n
1.4. n n)=1
1.5. (n r)=(n n-r)
2. 重复字母排列问题
2.1. AABBC has 5!/2!/2! ways
3. 乘法原理
3.1. 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.
3.2. 2*3*2 is the ways of getting from A to D
4. 排列
4.1. p(n,r)=n!/(n-r)!
4.1.1. MATHC choose 3
4.1.2. p(5,3)=5!/(5-3)!