A(3，2)=3×2，

When writing, 3 on the left side of the equal sign is subscript, 2 is superscript, and the number on the right side of the equal sign is multiplied by the number of superscript 2 from subscript 3, and each number is smaller than the previous one 1.

C (3,2) = (3× 2) ÷ (2×1) = 3, or c (3,2) = 3! ÷2! ÷(3-2)! =(3×2)÷(2× 1)÷ 1=3,

When writing, 3 on the left of the equal sign is subscript and 2 is superscript. The numerator on the right side of the equal sign is continuously multiplied by superscript 2 digits starting from subscript 3, and each digit is smaller than the previous digit by 1; the denominator is continuously multiplied by superscript 2 digits starting from superscript 2, and each digit is smaller than the previous digit by1; Or use the factorial of superscript, divided by the factorial of the subject below and then divided by the factorial of the difference between superscript and subscript.

Extended data

Permutation and combination is the most basic concept of combinatorics. The so-called arrangement refers to taking out a specified number of elements from a given number of elements for sorting. Combination refers to taking out only a specified number of elements from a given number of elements, regardless of sorting.

The central problem of permutation and combination is to study the total number of possible situations in a given permutation and combination. Permutation and combination are closely related to classical probability theory.

Permutation and combination formula and binomial theorem formula;

The two principles of addition and multiplication are the laws that run through. What has nothing to do with order is combination, but what needs order is arrangement. ?

Two formulas, two properties, two ideas and methods. Arrange and combine the summary, and the application questions must be transformed.

It is common sense to arrange and combine together and choose the back row first. Special elements and positions should be considered first.

Don't worry too much, and don't miss too much. Punching is a skill. Arrange combinatorial identities and define proof modeling tests.

On binomial theorem, China Yang Hui Triangle. Two properties, two formulas, function assignment transformation.

Resources Baidu Encyclopedia. permutation and combination