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Formula of permutation number: (the first letter in the registration indicates the superscript of the letter A (or C), and the second letter indicates its subscript)

1. Multiplication expression: a (m, n) = n (n-1) (n-2) ... (n-m+1). m ≤ n，

For example, a (2,4) = 4 * 3 =12.

2. factorial expression: A(m, n)=n! /(m-n)！

For example, a (2,4)) = 4! /(4-2)! =4! /2! .

=4*3*2* 1/(2* 1).

=4*3.

= 12.

Combination number formula:

1. Multiplication expression: c (m, n) = p (m, n)/p (m, m) = n (n-1) (n-2) ... (n-m+1)/m! .

C(m，n)=n(n- 1)(n-2)...(n-m+ 1)/[m(m- 1)(m-2)...2* 1];

For example, C(2.4)=4*3/2* 1.

=6.

2. factorial expression: C(m, n)=n! /[m！ *(n-m)！ ]

For example, g (2,4) = 4! /[(2! )*(4-2)!

=4! /(2! *2! ).

=4*3*2* 1/[(2* 1)*(2* 1)] .

=3*2.

=6.

I wish you academic progress and a happy Lantern Festival!