Advanced Geometric Problem & Solution (Twice a week update) Now 4779 problems posted.
<Qua-2026-816>, hint
AC and BD intersect at O. Let OA=a, OB=b, OC=c, OD=d, <AOB=X
By cosine 2nd law and given condition you can get
(ab+bc+cd+da) cos X=0, cosX=0, X=90, ACㅗBD
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