(1) arrangement, generally speaking, m(m≤n) elements are taken out of n different elements and arranged in a column according to a certain order, which is called the arrangement of taking m elements out of n elements.

(2) Combination is a mathematical term. Generally speaking, taking m(m≤n) elements from n different elements as a group is called taking out the combination of m elements from n different elements.

Second, the calculation method is different:

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

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

For example:

( 1)A(4，2)=4！ /2! =4*3= 12

(2)C(4，2)=4！ /(2! *2! )=4*3/(2* 1)=6

Extended data:

Difficulties in permutation and combination:

(1) It requires a strong ability of abstract thinking to abstract several concrete mathematical models from various practical problems.

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

(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) Whether the calculation scheme is correct can't be tested by intuitive methods, which requires us to understand the concepts and principles and have strong analytical ability.