Given a tetrahedron , let , , and . Prove that there exists a triangle with edges
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Given a tetrahedron $ABCD$, let $x = AB \cdot CD$, $y = AC \cdot BD$, and $z = AD \cdot BC$. Prove that there exists a triangle with edges $x, y, z.$
Source: Međunarodna matematička olimpijada, shortlist 1973
Školjka 
, let 
, 
, and 
. Prove that there exists a triangle with edges 