Here's a problem my friend gave me:(adsbygoogle = window.adsbygoogle || []).push({});

The number of points of intersection of diagonals of a n-gon is 70. If no three diagonals are concurrent, find the number of sides of the n-gon.

I believe the answer is 20.

I tried to work out for small values of n to get a feel but that didn't do much good. As n increases so does the number of intersection points but how to set up an expression relating this number to n? Suggestions are appreciated.

Thanks

Vivek

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# Homework Help: Combinatorics problem

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