Interior Angle

An interior angle of a polygon is the angle inside the polygon between two adjacent sides at a polygon vertex. For a convex polygon with sides, the sum of the interior angles is

Consequently, every interior angle of a regular polygon is

At each vertex of a convex polygon, the interior angle and the exterior angle formed by extending an adjacent side are supplementary angles. A second convention defines the exterior angle as the reflex angle outside the polygon; see exterior angle.