In primary school mathematics, five numbers are divided into two groups, the first group is four numbers, and the second group is 1 number. That is to say, when the number of 1 of the second group is determined, the number of the first group is determined. Since the second group number * * * has five combinations, the first group number also has five combinations.
For example, 12345 has five numbers, and 1 has four groups.
When the second group is 1, the first group has 2345.
When the second group is 2, the first group is 1345.
And so on.
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 a lot of thinking is needed when choosing the correct and reasonable calculation scheme.
The calculation method of permutation and combination is as follows:
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