Tangent Points on a Semicircle

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Let AB be the diameter of a semicircle. Let CD and CE be tangents from a point C. Let the intersection of AE and BD be F. Then CF is perpendicular to AB.


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Contributed by: Jay Warendorff (March 2011)
After work by: Antonio Gutierrez
Open content licensed under CC BY-NC-SA



The statement of the theorem is in Archimedes' Book of Lemmas: Proposition 12.

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