Permutation and combination is the most basic concept of combinatorics. 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. The c you said in the question is the combination of permutation and combination, in no particular order.

1, the definition of arrangement: from n different elements, any M (M ≤ N, m and n are natural numbers, the same below) elements are arranged in a column in a certain order, which is called an arrangement of taking m elements out of 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).

Calculation formula:

2. Definition of combination: randomly selecting m(m≤n) elements from n different elements for grouping, which is called taking out the combination of m elements from n different elements; The number of all combinations of m(m≤n) elements from n different elements is called the number of combinations of m elements from n different elements. Represented by the symbol C(n, m).

Calculation formula:

C(n，m)=C(n，n-m).(n≥m)

Besides, the rule is 0! = 1(n！ It means n(n- 1)(n-2)... 1, which is 7! =7x6x5x4x3x2x 1