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連接AO,BO OA=OC+CA OA+OB+CO=0 所以OC+CA+OB+CO=0 CA+OB=0 所以|CA|=|BO| 同理|CB|=|AO| 又|AO|=|BO|=|CO| 所以△AOC和△BOC都是等邊三角形 所以∠C=60°+60°=120°
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