Solution:
C(3,2)=3*2/2* 1=3
Or C(3,2) = c (3, 1)=3.
Basic counting principle and application of two commonly used arrays
1, addition principle sum classification counting method;
Each method in each class 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:
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.