IMO Shortlist 1981 problem 18
Dodao/la:
arhiva2. travnja 2012. Several equal spherical planets are given in outer space. On the surface of each planet there is a set of points that is invisible from any of the remaining planets. Prove that the sum of the areas of all these sets is equal to the area of the surface of one planet.
%V0
Several equal spherical planets are given in outer space. On the surface of each planet there is a set of points that is invisible from any of the remaining planets. Prove that the sum of the areas of all these sets is equal to the area of the surface of one planet.
Izvor: Međunarodna matematička olimpijada, shortlist 1981
Školjka 
