A decagon is a ten-sided, closed-plane figure with eight triangles in it. These eight triangles are formed by joining any vertex of the decagon to any other vertex. Thus, the triangles are formed by drawing the diagonals of the decagon.
The general formula for finding the number of triangles that can be formed is "n-2," where “n” is the number of sides of the polygon.
The sum of the interior angles of a decagon is 1440 degrees, and the exterior angle of a decagon is 36 degrees. The interior angle of a decagon is 144 degrees. It has 35 distinct diagonals that can be drawn for all vertices of the decagon.