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?
A B C D ABCD AC BD O A C BD B D AC E F G H ABCD ABCD
EFGH ABCD
AC BD EF HC BD AC EF HG AE=AF=CG=CH A C EF HG B D FG EH AOB AFB AF=OB=FB=DA 1/2AO*BO 1/2AF*BF EFGH AOB EFGH ABCD