1, areas overlap.

2. The words "most", "least", "most" and "at least" appear.

The minimum value of both: a ∩ b = the minimum value of a+b-I.

Minimum value of the three: minimum value of A∩B∩C = a+b+c-2i.

General application software

There are 500 students in a school, of whom 359 take math and 408 take literature. How many students have taken two courses?

165

Answer C. After reading the topic, we can judge the inclusion and exclusion of the two, but it involves the problem of seeking the least, so what is required is the extreme value problem. We can solve the extreme value problem through reverse thinking, and the topic requires at least two courses, that is, the number of people who have not selected courses is the largest.