The Exterior Angle Theorem
A Complete SAT Math Guide — From Basics to Mastery
Introduction — Key Vocabulary
What Is an Exterior Angle?
An exterior angle is the angle formed by one side of a polygon and the extension of an adjacent side.
Think of it this way: pick any vertex of a triangle, then extend one of the two sides that meet there beyond the vertex. The angle between that extension and the other side is the exterior angle.
Interior vs. Exterior
- Interior angles — angles inside the polygon at each vertex.
- Exterior angles — angles formed outside the polygon by extending a side.
- Adjacent interior angle — the interior angle that shares a vertex and a side with the exterior angle.
- Remote (non-adjacent) interior angles — the two interior angles of the triangle that are NOT adjacent to the given exterior angle.