Nagel's Theorem
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Let ABC be a triangle with two altitudes AA' and BB', and let O be the center of the circumcircle. Then A'B' and CO are perpendicular.
Contributed by: Jay Warendorff (March 2011)
After work by: Alexander Bogomolny
Open content licensed under CC BY-NC-SA
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A. Bogomolny, "Nagel's Theorem," Interactive Mathematics Miscellany and Puzzles.
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