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gluing a quadrilateral pyramid with a regular tetrahedron of equal edges

Source: Flanders Math Olympiad 2008 p3

December 24, 2022
geometry3D geometrytetrahedronpyramid

Problem Statement

A quadrilateral pyramid and a regular tetrahedron have edges that are all equal in length. They are glued together so that they have in common 11 equilateral triangle . Prove that the resulting body has exactly 55 sides.