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Cut the Knot
Cut the Knot!: Monge's Theorem of Three Circles and Common Tangents
Very good site presenting a Trig problem dealing with tangents lines and circles, then linking the student to the solution
ClassFlow
Class Flow: Circles Geometry
[Free Registration/Login Required] Students identify segments and lines related to circles and the properties of a tangent to a circle.
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...