C (3,5) (upper 3, lower 5)= 5×4×3/(3×2× 1)

The expression means to choose three out of five people, and how many choices there are.

Probability theory, an algorithm of numbers up and down in C: Cmn=m! Du /[n! *(m-n)！ ] ? M down, n up! The factorial of n =1* 2 * 3 * * n.

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 the arrangement of M elements in N different elements, and is represented by the symbol A(n, m).

Extended data:

The arrangement formula is to build a model and arrange m different elements in a column (orderly). The first position can have n choices, and the second position can have n- 1 choices (1 has been put in the previous position). Similarly, we can know that the third position can have n-2 choices, and so on, and the m-th position can have n-m+.

Replacement with repetition is a special arrangement. You can repeatedly select m elements from n different elements. Arranging in a column in a certain order is called a repeatable arrangement of m elements starting from n elements. If and only if the selected elements are the same and the arrangement order of the elements is the same, the two arrangements are the same.

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