Triangle Angle Sum

Don't believe everything that you hear — make them prove it! The video shows two methods to prove the Triangle Angle Sum Theorem. The following two videos apply the theorem to finding missing angle measures.

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CCSS: Adaptable
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Instructional Ideas

  • Use the introduction in the video as a class activity; ask pupils to draw a triangle, cut out the angles, and predict the sum of the three angles
  • Post the video for future reference or those needing additional instruction
Classroom Considerations

  • Assumes knowledge of the angle relationships when a transversal cuts parallel lines 

  • Provides both a physical and a conceptual model to prove the Triangle Sum Theorem

  • Uses incorrect notation when referring to the measures of angles
Common Core