21

Part of 1989 IMO Shortlist

Problems(1)

Regular tetrahedron can be an obtuse-angled triangle

Source: IMO Shortlist 1989, Problem 21, ILL 67

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

120^{\circ}.

geometry3D geometrytetrahedronanglegeometric inequalityIMO Shortlist