The figure below shows a cyclic
quadrilateral ABCD. ABEF, BCGH, CDJK, and ADLM are rectangles of centers O1,
O2, O3, and O4, respectively, so that AF = CD, BH = AD, CK = AB,
and DL = BC. Prove
that O1O2O3O4 is a
rectangle.
See also:
Typography of problem 868.
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