Let Ox,Oy,Oz be three rays, and G a point inside the trihedron Oxyz. Consider all planes passing through G and cutting Ox,Oy,Oz at points A,B,C, respectively. How is the plane to be placed in order to yield a tetrahedron OABC with minimal perimeter ? geometry3D geometrytetrahedronperimeterminimizationIMO Shortlist