The exterior angle of a triangle
An exterior angle equals the sum of the two opposite interior angles.
The common mistake
Adding all three interior angles, or using the adjacent interior angle.
Instead: Add only the two interior angles not touching the exterior angle.
Worked example with Guide me
Interior angles 50° and 60° are opposite an exterior angle. Find it.
- Sugar · Which angles do we add?Student · The two opposite ones, 50° and 60°.
- Sugar · So?Student · 110°.
Answer: 110°
Try 3 practice questions
1. Opposite angles 35° and 80° — exterior angle?
Show answer
115°
2. Exterior 120°, one opposite angle 45° — the other?
Show answer
75°
3. Adjacent interior angle 70° — exterior angle?
Show answer
110°
Quick answers
- What is the most common mistake with the exterior angle of a triangle?
- Adding all three interior angles, or using the adjacent interior angle.
- How do I get the exterior angle of a triangle right?
- Add only the two interior angles not touching the exterior angle.
- Can you show a worked example?
- Interior angles 50° and 60° are opposite an exterior angle. Find it. 110°
Related concepts
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