4.2Triangle Angle Sum Theorem
Quick Answer
Triangle Angle Sum Theorem: the sum of the interior angles of any triangle is always 180°. Use this to find a missing angle: missing angle = 180° − (sum of the other two angles).
Essential Question
Why does the sum of the interior angles of every triangle equal 180°, and how do we use this to find missing angles?
Learning Goals
- 1State the Triangle Angle Sum Theorem.
- 2Find missing angle measures in triangles.
- 3Apply the theorem to multi-step problems.
- 4Prove the Triangle Angle Sum Theorem using parallel lines.
Key Vocabulary
Triangle Angle Sum Theorem
The sum of the measures of the interior angles of a triangle is 180°.
Interior angle
An angle formed inside a polygon by two of its sides.
Full lesson content coming soon
Worked examples, guided practice problems, a workbook, quiz, and downloadable PDF resources will be added to this lesson in Phase 2. The learning goals and vocabulary above reflect the complete lesson scope.