exterior angle 
noun Geometry
The angle between a side of a rectilinear figure and an adjacent side extended outward.Also the Pythagoreans knew the generalisation which states that a polygon with n sides has sum of interior angles 2n - 4 right angles and sum of exterior angles equal to four right angles....
- In any triangle, if one of the sides be produced, the exterior angle is greater than either of the interior and opposite angles.
- At each vertex of a triangle there are a couple of angle bisectors: a bisector of the interior angle and a bisector of the exterior angle.