SOLUTION: Regular polygon has 20 sides. How many triangles can be formed using these vertices but,not using the sides?
Ans.800
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Question 835852: Regular polygon has 20 sides. How many triangles can be formed using these vertices but,not using the sides?
Ans.800
Answer by richard1234(7193) (Show Source): You can put this solution on YOUR website!
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. This makes 
triangles we have to subtract, since one of the side lengths is on the 20-gon.
1140 - 340 = 800.
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