IMO Shortlist 1984 problem 5
Dodao/la:
arhiva2. travnja 2012. Prove that

, where

and

are non-negative real numbers satisfying

.
%V0
Prove that $0\le yz+zx+xy-2xyz\le{7\over27}$, where $x,y$ and $z$ are non-negative real numbers satisfying $x+y+z=1$.
Izvor: Međunarodna matematička olimpijada, shortlist 1984