Vertex Angle
The point about which an angle is measured is called the angle's vertex, and the angle 
associated
with a given vertex is called the vertex angle.
In a polygon, the (interior, i.e., measured on the interior side of the vertex) are generally denoted 
or 
. The sum of
interior angles in any 
-gon is given by 
radians,
or 
(Zwillinger 1995, p. 270).
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