Is equal to a fraction, the numerator is n*(n- 1), and the denominator is m, multiplied by 1.

Select m elements from n elements (m

Combination is one of the important concepts in mathematics. Taking out M different elements (0≤m≤n) from N different elements at a time and synthesizing a group regardless of their order is called selecting the combination of M elements from N elements without repetition.

The relationship between elements and sets

There are only two possibilities between element a and a given set a:

1, A belongs to the set A, which is expressed as an element of the set A and recorded as A ∈ A

2.A does not belong to the set A, which means that A is not an element of the set A, and it is recorded as A? A