For example: A (4 4,2) = 4! /2! =4*3= 12。
The formula of combination: C(n, m)=P(n, m)/P(m, m) =n! /m！ *(n-m)！ .
For example: c (4 4,2) = 4! /(2! *2! )=4*3/(2* 1)=6。
Addition principle sum classification counting method
1, addition principle: There are n ways to do something. In the first way, there are m 1 different ways, in the second way, there are m2 different ways, ..., in the n ways, there are mn different ways, so there are N = m 1+M2+ to complete it.
2. The method of the first method belongs to the set A 1, the method of the second method belongs to the set A2, ..., and the method of the n method belongs to the set An, so the method to accomplish this belongs to the set A 1ua2u...uan.
3. Classification requirements: each method in each category can accomplish this task independently; The specific methods in the two different methods are different from each other (that is, the classification is not heavy); Any method to accomplish this task belongs to a certain category (that is, classification does not leak).