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).