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Different Proof of the Exterior Angle Theorem

June 25, 2010

This isn’t the point of Jason’s post (although he makes a great point about confusing notation), but I had forgotten a different proof of the exterior angle theorem:

The idea is that at if a line is drawn at the vertex that is parallel to the side opposite, it divides the exterior angle into two angles each of which is congruent to one of the remote interior angles (alternate interior angles and corresponding angles). Pretty cool proof!

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