The figure below shows a cyclic
quadrilateral ABCD. ABEF, BCGH, CDJK, and ADLM are rectangles of centers O_{1},
O_{2}, O_{3}, and O_{4}, respectively, so that AF = CD, BH = AD, CK = AB,
and DL = BC. Prove
that O_{1}O_{2}O_{3}O_{4} is a
rectangle.
See also:
Typography of problem 868.
