Exterior Angles of Polygons

The Exterior Angle is the angle between any side of a shape, and a line extended from the next side.

external image interior-exterior-angles.gif
Note: If you add up the Interior Angle and Exterior Angle you get a straight line, 180°. (See Supplementary Angles)exterior angle theorem

The Exterior Angles of a Polygon add up to 360°

In other words the exterior angles add up to one full revolution
Think of it this way: the lines change direction and eventually return back to the start.
(Exercise: try this with a square or some odd-shaped polygon)

Note: This rule only works for simple polygons

external image interior-angles-pentagon.gif