# Three Planes in a Trihedron

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.

O

OA

OB

OC

u>1

OA

OB

OC

ABC'

ACB'

BCA'