Regular tetrahedron can be an obtuse-angled triangle
Source: IMO Shortlist 1989, Problem 21, ILL 67
September 18, 2008
geometry3D geometrytetrahedronanglegeometric inequalityIMO Shortlist
Problem Statement
Prove that the intersection of a plane and a regular tetrahedron can be an obtuse-angled triangle and that the obtuse angle in any such triangle is always smaller than