The formula of high school arrangement and combination is: C(n, m)=A(n, m)/m! =n！ /m！ (n-m)！ And C(n, m)=C(n, n-m).

For example, c (4 4,2) = 4! /(2! *2! )=4*3/(2* 1)=6，C(5，2)=C(5，3)。

The calculation method of permutation and combination C: C is selected from several, there is no permutation, only combination. ?

C(n，m)=n*(n- 1)*...*(n-m+ 1)/m！

For example, C53 = 5 * 4 * 3÷ (3 * 2 *1) =10, and c (4 4,2) = (4x3)/(2x1) = 6.

Two common basic counting principles of permutation and their applications;

1, addition principle sum classification counting method;

Each method in each category can accomplish this task independently. The specific methods in two different categories are different from each other (that is, the classification is not heavy), and any method to accomplish this task belongs to a certain category (that is, the classification is not leaking).

2. Multiplication principle and step counting method:

One method of any step can't complete this task, and this task can only be completed by continuously completing these n steps, and the counting of each step is independent of each other. As long as the methods used in a step are different, the corresponding methods to complete it are also different.