MEMO 2011 ekipno problem 5
Dodao/la:
arhiva28. travnja 2012. Let

be a convex pentagon with all five sides equal in length. The diagonals

and

meet in

with

. Prove that

has a pair of parallel sides.
%V0
Let $ABCDE$ be a convex pentagon with all five sides equal in length. The diagonals $AD$ and $EC$ meet in $S$ with $\angle ASE = 60^\circ$. Prove that $ABCDE$ has a pair of parallel sides.
Izvor: Srednjoeuropska matematička olimpijada 2011, ekipno natjecanje, problem 5