Centagon
Probability
Level
4
Find the number of rectangles that can be formed by joining 4 vertices of a 100-sided regular polygon.
The answer is 1225.
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1 solution
Each pair of diametrically opposite vertices forms a diagonal of a rectangle. There are ( 2 5 0 ) = 1 2 2 5 pairs of diagonals that can be chosen, and so that number of rectangles.