The arrangement A(n, m)=n×(n- 1). (n-m+ 1)=n！ /(n-m)！ (n is subscript and m is superscript, the same below).

Combination C(n, m)=P(n, m)/P(m, m)=n! /m！ (n-m)！ .

For example, a (4,2) = 4! /2! =4*3= 12。

C(4，2)=4！ /(2! *2! )=4*3/(2* 1)=6。

Addition principle sum distribution counting method;

1, addition principle: There are n ways to do one thing. 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+M3+ to complete.

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 AUA2....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).