There is a theorem about the sum of the external angles of a polygon. Check it out.
http://www.mathwarehouse.com/geometry/polygon/index.php
Draw three different nonconvex polygons. When you walk
around a polygon, at each vertex you need to turn either
right (clockwise) or left (counterclockwise). A turn to the
left is measured by a positive number of degrees and a turn
to the right by a negative number of degrees. Find the sum
of the measures of the turn angles of the polygons you
drew. Assume you start at a vertex facing in the direction
of a side, walk around the polygon, and end up at the same
vertex facing in the same direction as when you started.
2 answers
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