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Three math books
Regardless of the differences between books, we can choose five of the nine books, and there are 95 ways to choose.

There is no math book, that is, there is a C65 way to take any five of the other six books.

Indirect method, * * has C95-C65= 120 methods.

Basic counting principle and application of two commonly used extended data arrangements

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.