Is there a good source on the Web for proofs of basic geometric theorems?

I'm a geometry tutor, and I need to find a proof for the following theorem: If a tangent segment and a secant segment are drawn to a circle from an exterior point, then the square of the measure of the tangent segment is equal to the product of the measures of the secant segment and its external secant... show more I'm a geometry tutor, and I need to find a proof for the following theorem:

If a tangent segment and a secant segment are drawn to a circle from an exterior point, then the square of the measure of the tangent segment is equal to the product of the measures of the secant segment and its external secant segment.

Ideally, I'd like to find one website or resource that compiles proofs of every geometric theorem that a high school student might encounter.
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