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Contrary to intuition, the external angle of a vertex of a polygon is not the size of the angle between the sides forming that vertex, as measured outside the polygon.
An external angle is in fact an angle formed by one side of a polygon and a line produced from an adjacent side.
While $\angle AFE$ is the internal angle of vertex $F$, the external angle of this vertex is $\angle EFG$.
Also known as
An external angle is also known as an exterior angle.
- Vertical Angle Theorem, which shows that it does not matter which adjacent side you use, because they are equal.
- Results about external angles can be found here.
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): polygon