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.