Possible Duplicate:
Prove that CX and CY are perpendicular
There is given convex quadrilateral ABCD. And internal bisectors of angle ∠A and ∠C intersect in point X. And internal bisectors of angle ∠B and ∠D intersect in point Y. Prove that if AY is is the external bisector of A then CY is the external bisector of C.
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