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:

Basic counting principle in permutation and combination

1, addition principle sum classification counting method

(1) addition principle: There are n ways to do one thing, and finish it. In the first way, there are m 1 different ways, in the second way, there are m2 different ways, ... and in the n ways, there are mn different ways, so there are n = M 1+M2 to complete it.

(2) The method of the first method belongs to the set A 1, the method of the second method belongs to the set A2, ..., and the method of the n method belongs to the set An, so the method to accomplish this belongs to the set A 1ua2u...uan.

(3) Classification requirements: each method in each category can accomplish this task independently; The specific methods in the two different methods are different from each other (that is, the classification is not heavy); Any method to accomplish this task belongs to a certain category (that is, classification does not leak).

2. Multiplication principle and step counting method

(1) Multiplication principle: To do one thing, it needs to be divided into n steps. There are m 1 different ways to do the first step, m2 different ways to do the second step, ... and mn different ways to do the nth step, so there is n = m 1× m2× m3× to complete it.

(2) Reasonable step-by-step requirements

One method of any step can't complete this task, and only by continuously completing these n steps can this task be completed; Each step is independent of each other; As long as the methods used in a step are different, the corresponding methods to complete it are also different.