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1)因為∠BCA為圓周角,囙此∠BCA=90度=> BC平行OD =>∠BDO =∠CBD因為∠BCA=90度,∠BAC=30度,囙此∠CBA=60度因為OB = OD,囙此△OBD為等腰△,∠BDO =∠BAC=>∠BDO =∠BAC =∠CBD = ½;∠CBA = 30度2)Rt△A…
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