Exterior angle of a triangle
An exterior angle is formed when one side of a triangle is extended. It is linked to the two non-adjacent interior angles, which are the two interior angles not touching that exterior angle.
Method
- Identify the exterior angle at the extended side.
- Find the two remote interior angles inside the triangle.
- Add those two angles.
- Simplify the result if the answer involves expressions.
Why this works
The exterior angle of a triangle equals the sum of the two remote interior angles. This lets you find an unknown angle quickly when the other two are known.
Check your answer
- The exterior angle should be larger than either of the two remote interior angles.
- It should also match the triangle’s angle relationships: the interior angle next to it and the exterior angle form a straight line, so they add to 180°.
Tips
- Be careful not to use the two interior angles that touch the exterior angle.
- If the problem gives algebraic expressions, combine like terms before writing the final simplified answer.