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

Comments on permutation and combination:

For some permutation and combination problems that need to be adjacent, the adjacent elements can be regarded as a "meta" arranged with other elements, and then the interior of the "meta" can be arranged. Note: For the arrangement of some nonadjacent elements, we can arrange other elements first, and then insert nonadjacent elements between the arranged elements and the gaps at both ends.