An heptagon has 7 sides and 14 diagonals
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38 diagonals
In a polygon with n sides, we have n(n-3)/2 diagonals. In a convex polygon with n sides, you can draw n-3 diagonals from each vertex, but you are counting each one twice you so you need to divide by do. That is why we have n(n-3) divided by 2
9
18
20 sides 1/2*(202-60) = 170 diagonals