IMO Shortlist 1962 problem 7
Dodao/la: arhiva2. travnja 2012.
has the following property: there exist five spheres, each tangent to the edges
or to their extensions.
a) Prove that the tetrahedron
b) Prove conversely that for every regular tetrahedron five such spheres exist.
Izvor: Međunarodna matematička olimpijada, shortlist 1962