Three Planes in a Trihedron
Start with a trihedron with apex and edges , , . Extend the edges by a factor to a trihedron , , . Then the planes , and are concurrent. The common point is shown in red.
This problem was posed in [1, p. 220].
 V. V. Prasolov and I. F. Sharygin, Problems in Stereometry (in Russian), Moscow: Nauka, 1989.