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Calculation formula of permutation and combination problem
The calculation method of permutation and combination is as follows: permutation can also be expressed as p.

The arrangement a (n, m) = n× (n- 1). (n-m+ 1) = n! /(n-m)! (n is subscript and m is superscript, the same below)

Combination C(n, m)=P(n, m)/P(m, m) =n! /m! (n-m)! ;

For example:

A(4,2)=4! /2! =4*3= 12

C(4,2)=4! /(2! *2! )=4*3/(2* 1)=6

The difference between c and p in probability;

1 means different.

C stands for combination, such as A, B and C. There are three ways to take out two people to participate in activities, namely A, B, C and B, which are not sequential, but only combination.

P stands for arrangement method, which means to arrange some objects in order. What is the general method?

2. Different in nature

Formula P refers to arrangement, and R elements are selected from N elements for arrangement (i.e. sorting).

Formula C refers to combination, in which R elements are taken from N elements without arrangement (that is, without sorting).

Extended data

Difficulties in permutation and combination:

1. Abstracting several concrete mathematical models from different practical problems requires strong abstract thinking ability;

2. Restrictive conditions are sometimes obscure, which requires us to accurately understand the key words in the question (especially logical related words and quantifiers);

3. The calculation method is simple and has little connection with the old knowledge, but it needs a lot of thinking when choosing the correct and reasonable calculation scheme;

4, the calculation scheme is correct, often can't use intuitive method to test, requires us to understand the concept, principle, and have strong analytical ability.