An exterior angle 
of a polygon is the angle formed externally between two adjacent sides. It is therefore
equal to 
,
where 
is the corresponding internal angle between two adjacent sides (Zwillinger 1995,
p. 270).
Consider the angles 
formed between a side of a polygon and the extension of an adjacent side. Since there
are two directions in which a side can be extended, there are two such angles at
each vertex. However, since corresponding angles are opposite, they are also equal.
Confusingly, a bisector of an angle 
is known as an exterior
angle bisector, while a bisector of an angle 
(which is simply a line oriented in the opposite direction
as the interior angle bisector) is not given any
special name.
The sum of the angles 
in a convex polygon is equal to 
radians (
), since this corresponds to one complete rotation
of the polygon.
