A is arrangement, which is related to order. C is a combination, in no particular order. The so-called arrangement refers to taking out a specified number of elements from a given number of elements for sorting. Combination refers to taking out only a specified number of elements from a given number of elements, regardless of sorting.

Definition of arrangement: randomly select M different elements from N different elements and arrange them into a column in a certain order, which is called the arrangement of M elements among N different elements; All permutation numbers of m(m≤n) elements taken from n different elements are called permutation numbers of m elements taken from n different elements, which are represented by symbol A(n, m). The calculation formula of A(n, m) is as follows:

Definition of combination: taking any m(m≤n) elements from n different elements to form a group is called taking the combination of m elements from n different elements; The number of all combinations of m(m≤n) elements taken out of n different elements is called the number of combinations of m elements taken out of n different elements, which is represented by symbol C(n, m). The calculation formula of C(n, m) is as follows:

Or C(n, m)=A(n, m)/A(m, m).

If this problem is directly understood as dividing by 2, it is not convenient to remember the formula. Coincidentally, the factorial of 2 is 2! Equal to 2, best written as factorial divided by 2.