Exterior Angle
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.
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