It has 10 sides because using the formula 0.5*(102-30) = 35 diagonals
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It depends if the polygon is convex or concave but if it is a regular polygon it would have 560
54 Diagonals. * * * * * A polygon with n sides has 1/2*n*(n-3) diagonals. So a decagon would have 1/2*10*7 = 35 diagonals. How the Community answer got 54 is anybody's guess!
A diagonal line of a polygon is a line that joins any two vertices not already joined by a side. A polygon with n sides has n(n-3)/2 diagonals → a decagon has 10(10-3)/2 = 10 × 7 ÷ 2 = 35 diagonals
A decagon has 10 sides and 35 diagonals
An octagon has 8 sides and 20 diagonals so the question doesn't make sense