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How many combinations are there?
* * * There are five combinations, and the solution of high school mathematics is C=5.

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