A regular octadecagon (an eighteen sided polygon) would have nine diagonals.
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It has 9 sides and it can be an regular or a irregular polygon which is called a nonagon Check: 0.5*(81-27) = 27 diagonals
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9
There are 9 diagonals in any hexagon!The hexagon does not have to be regular.
A polygon that has 104 diagonals will have 16 sides