Let ABCD be a parallelogram and H the Orthocentre of △ABC. The line parallel to AB through H intersects BC at P and AD at Q while the line parallel to BC through H intersects AB at R and CD at S. Show that P, Q, R and S are concyclic.(Swiss Mathematical Olympiad 2011, Final round, problem 8) geometryparallelogramratiosimilar trianglespower of a pointgeometry proposed