A(n，m)=n*(n- 1)*(n-2)*...*(n-m+ 1)，？ A(n, m) is to multiply the first number n in brackets with the number before it, multiply the first number m in the direction of 1, and how many numbers to multiply when m is a number.

For example:

A(n，5)= n *(n- 1)*(n-2)*(n-3)*(n-4)

A(n，4)=n*(n- 1)*(n-2) *(n-3)

A(n，3)=n*(n- 1)*(n-2)

Extended data:

A(n, m) is a combinatorial arrangement formula, which means to select m numbers from n different numbers and combine them to see how many methods there are. If the numbers are the same but in different order, the methods will be different. ?

Because m is selected from n numbers, it should be noted that m is less than or equal to n.