A(m, n)m is at the bottom and n is at the top, which means that N elements selected from M elements are arranged in a certain order.

C(m, n)m is at the bottom and n is at the top, which means that N elements are selected from M elements for combination.

Calculation of c:

The number of subscripts is multiplied by the number of themes, and each number must be-1. And then divided by the factorial of the subject.

For example: C5 3 (subscript 5, superscript 3)=(5X4X3)/3X2X 1.

3X2X 1 (i.e. factorial of 3)

Calculation of a:

Just like the first step of C, there is no need to divide the factorial of the above target.

For example A4 2 = 4X3? .