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連接BC.BD.過D作DE垂直AC於.因為角DAB和角DCB是同弦的兩個園周角(互補)且ACB=90(直徑的園周角)CAB=30.DAC=DCA(等腰)所以2DAC+BAC+ACB=180.解得DAC=30.然後再跟據三角形ADE三邊比算.
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