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.

Concepts

triangle sum theorem, interior angles, geometric proofs

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Additional Tags

math

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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 

Pros

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

Cons

• Uses incorrect notation when referring to the measures of angles

Common Core

HSG-CO.C.10