The figure below shows a cyclic
quadrilateral ABCD. ABEF, BCGH, and CDJK are rectangles of centers O_{1},
O_{2}, and O_{3}, respectively, so that AF = CD, BH = AD, and CK = AB. Prove
that the measure of angle O_{1}O_{2}O_{3} is 90 degrees.
See also:
Typography of problem 866.
