1/2*15*(15-3) = 90 diagonals.
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There are 44 diagonals in an 11 sided undecagon
There are: 0.5*(302-(3*30)) = 405 diagonals
104 16x ((16-3)/2) =104
22-gon
A n sided figure has 1/2 n(n - 3) diagonals → a 17-gon has 1/2 x 17 x (17 - 3) = 1/2 x 17 x 14 = 119 diagonals