There are several books on history, literature and popular science in the corner of the classroom. Each student can borrow two books at will. So, how many students should there be at least?

This is actually a combination of three plus two, that is, three objects, two at a time. How many ways to take it, plus 1, is what you want.

C(2，3)+C( 1，3)+ 1=3+3+ 1=7

Or use the exhaustive method: let three books be A, B and C, and take two books at a time. Then the possible methods are as follows:

There are six kinds of AA, BB, CC, AB, AC, BC and * * *. If there is another kind, it will definitely be repeated with one of the previous six kinds, so there must be something in common among seven people.

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