Another way to write c(n, k) is to select any number of combinations of k from n different objects.

Generally speaking, taking m(m≤n) elements from n different elements as a group is called taking out the combination of m elements from n different elements. We call the problem of finding the combination number the combination problem.

Extended data:

The number of combinations of m elements from n different elements = the number of combinations of (n-m) elements from n different elements.

This property is easy to understand, such as c (9,2) = c (9,7), that is, the method of selecting two elements from nine elements is equal to the method of selecting seven elements from nine elements.

Take out m different elements from N and arrange them in a row (orderly). The first position can have n choices, and the second position can have n- 1 choices (the previous position already has 1 choices). Similarly, it can be known that the third position can have n-2 choices, and so on, and the m-th position can have n-m+ 1 choices.

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