Two Conditions for a Tetrahedron to Be Orthocentric

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An altitude of a tetrahedron is a line from a vertex perpendicular to the face opposite that vertex. A tetrahedron is orthocentric if the four altitudes meet at the same point, which is called the orthocenter or the Monge point.
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Contributed by: Izidor Hafner (April 2017)
Open content licensed under CC BY-NC-SA
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Details
The proof can be found in [1, p. 123].
Reference
[1] V. V. Prasolov and I. F. Sharygin, Problems in Stereometry (in Russian), Moscow: Nauka, 1989.
Permanent Citation
"Two Conditions for a Tetrahedron to Be Orthocentric"
http://demonstrations.wolfram.com/TwoConditionsForATetrahedronToBeOrthocentric/
Wolfram Demonstrations Project
Published: April 25 2017