Two properties of combinatorial numbers are complementary properties and combinatorial identities. 1, complementary property. The number of combinations of m elements from n different elements = the number of combinations of (n-m) elements from n different elements. 2. Combinatorial identities. If m item is selected from n items, the following formula exists: c (n, m) = c (n, n-m) = c (n- 1, m- 1)+c (n- 1, m).

Combination is a very important knowledge point in mathematics learning. Combination is to take out m different elements from n different elements at a time, regardless of their order, and synthesize a group, which is called the combination of selecting m elements from n elements without repetition. The number of species in all such combinations is called the combination number.

In addition, the nature of the combination number also stipulates that c (n, 0) = 1 c (n, n) =1c (0,0) = 1. In the study of mathematics, we can deeply understand that mathematics comes from our daily life through the study of combination numbers, and the study of combination numbers makes mathematics concrete and vivid.