how many triangles are there in a 20-gon ??
what is the calculation ??

 

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There?are 20c3= 1140 ways to choose three vertices that form a?triangle. Of these,?there?are?20 triangles?with two side lengths on the perimeter of the regular polygon. Also, for each side,?there?are 16 points that can make a?triangle?with exactly one side length on the?20-gon.
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