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。
Extended data:
There are n ways to do something and finish it. The first way has m 1 different ways, and the second way has m*n different ways, so there are N = M 1+m2+M3+…+Mn different ways to accomplish it.
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.
Each method in each class 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.
Arrangement is related to the order of elements, and combination has nothing to do with order. For example, 23 1 and 2 13 are two permutations, and the sum of 2+3+ 1 is a combination.
Baidu Encyclopedia-permutation and combination (one of combinatorial mathematics)