Question

Find the number of diagonals and triangles formed in a decagon

Answer 1

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35

So there are 45 line segments joining pairs of vertices. Exactly 10 of those are sides of the decagon, the others are diagonals

8 triangles

If you are considering all the vertices independently you will have a total of 8*10 = 80 triangles. You can figure out this way: the sum of the interior angles of a decagon = (2n-4)*right angles or (n-2)*triangles. So a decagon will form 8 triangles.