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Khan Academy
Khan Academy: Inscribed Angle Theorem Proof
Proving that an inscribed angle is half of a central angle that subtends the same arc.
Other
Propagation of Light Using Geometry
A discussion of Christian Huygens' contribution to wave optics, and particularly to our understanding of the reflection and refraction of light. Excellent diagrams and a geometric proof on why the law of reflection is mathematically...
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.
Maths Challenge
Maths challenge.net: Equal Angles
A geometric proof. Prove two angles in this model are equal in measure.
Maths Challenge
Maths challenge.net: Unique Circle Equal Angles
A "three star geometry problem" - requiring the knowledge of biconditional statements and how to prove such problems.
Mangahigh
Mangahigh: Shape: Proof of the Sum of the Angles in a Triangle
Determine that all angles in a triangle equal 180 degrees.
Other
Antonio Gutierrez: Geometry From the Land of the Incas
Discovering the properties of constructions, seeing proofs in action, and viewing problems come to life here with this wonderful use of technology in the classroom. Single steps are explained and illustrated one at a time so that the...
Oswego City School District
Regents Exam Prep Center: Theorems and Postulates for Geometry
Here is a partial list of theorems, postulates, and properties.