Varignon's theorem states that the bimedians of a quadrilateral bisect each other (left figure).
In addition, the three bimedians of a tetrahedron are concurrent
and bisect each other (right figure; Altshiller-Court 1979, p. 48).
Altshiller-Court, N. Modern Pure Solid Geometry. New York: Chelsea, 1979.Neuberg, J. "Notes
Mathématiques: 49. Probléme sur les tétraèdres."
Mathesis38, 446-448, 1924.
