A decagon is a polygon that has 10 sides. Decagons can be both regular polygons, which have sides of equal length and equal interior angles between each side, or irregular polygons, which have unequal sides and angles between sides.
All the interior angles of a regular decagon add up to 1,440 degrees. The formula to find the value of an interior angle of a regular polygon is (180n-360)/n, with n being the number of sides. Using 10 as n, this formula shows that a regular decagon has interior angles of 144 degrees, making the exterior angles measure 36 degrees each.