IMO Shortlist 1966 problem 15


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2. travnja 2012.
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Given four points A, B, C, D on a circle such that AB is a diameter and CD is not a diameter. Show that the line joining the point of intersection of the tangents to the circle at the points C and D with the point of intersection of the lines AC and BD is perpendicular to the line AB.
Izvor: Međunarodna matematička olimpijada, shortlist 1966