At a class reunion, suppose every two people take a photo and the photographer takes 28 photos. How many students are there at the party?

Suppose there are x people in class one, then put them in order first. The first person takes pictures with X- 1, and the second person takes pictures with X-2. . . The penultimate person took a photo of 1, so a * * * has it.

1+2+...+(X-2)+(X- 1)=28

X(X- 1)/2=28 can be obtained after sorting.

x？ -X-56=0

Rank (X-8)(X+7)=0.

The solution is X=-7 (truncated)

X=8

To sum up, a * * * has 8 participants!

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