IMO 1979 LL GRE36
A regular tetrahedron A1B1C1D1 is inscribed in a regular
IMO 1979 LL GRE36
Origin: GRE
Problem
A regular tetrahedron A1B1C1D1 is inscribed in a regular tetrahedron ABCD, where A1 lies in the plane BCD, B1 in the plane ACD, etc. Prove that A1B1 \geqAB/3.