CK-12 Foundation
Proofs: Angle Pairs and Segments—The Three Angle Problem
Finding the sum of the measures of three angles is easy, unless you have no clue what the measures are. Learners use an interactive diagram to see a geometric problem in a different way. A set of challenge questions takes them through...
Mangahigh
Mangahigh: Shape: Proof of the Exterior Angle Theorem for a Triangle
Use your knowledge of exterior angles of a triangle to prove the theorem.
Mangahigh
Mangahigh: Shape: Proof of the Sum of the Angles in a Triangle
Determine that all angles in a triangle equal 180 degrees.