Analysis process:
18 = 2x9 = 6x3 = 9x2 =18x1or 9X2=3X6, a * * *, there are seven kinds of division methods.
The calculation method of permutation and combination is as follows:
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