Any solution will have to be divided by two.
How many diagonals does a 21 sided polygon have? Question: How many diagonals does a polygon with 18 sides have if three of its vertices, which are adjacent to each other, do not send any diagonals? The area, T, of one of the equilateral triangles, drawn in blue, can be found by using , where the apothem is the height of the triangle.
Since a diagonal is a line segment joining non-adjacent vertices, the shape of a diagonal is a straight line.