IMO Shortlist 2005 problem G3


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2. travnja 2012.
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Let ABCD be a parallelogram. A variable line g through the vertex A intersects the rays BC and DC at the points X and Y, respectively. Let K and L be the A-excenters of the triangles ABX and ADY. Show that the angle \measuredangle KCL is independent of the line g.
Izvor: Međunarodna matematička olimpijada, shortlist 2005