IMO Shortlist 1986 problem 20
Dodao/la:
arhiva2. travnja 2012. Prove that the sum of the face angles at each vertex of a tetrahedron is a straight angle if and only if the faces are congruent triangles.
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Prove that the sum of the face angles at each vertex of a tetrahedron is a straight angle if and only if the faces are congruent triangles.
Izvor: Međunarodna matematička olimpijada, shortlist 1986
Školjka 
