Question Number 157087 by henderson last updated on 19/Oct/21
$$ \\ $$Let ABCD be a tetrahedron, G, G' and G" the centers of gravity of faces ABC, ACD and ADB, I and J the midpoints of edges [BC] and [CD]. 1) Show that lines (IJ) and (GG' ) are parallel. 2) Prove that the points G, G' and G" are not aligned. 3) Show that the planes (GG 'G ") and (BCD) are parallel.