∵S△ACD∶S△ADB=1∶2∴S△ACD∶S△BCA=1∶3∵∠CAD=∠B，∠ACD=∠BCA∴△ACD∽△BCA∴S△ACD/S△BCA∴S△ACD/S△BCA=(DC/AC)²=1/3那么(3/AC)²=1/3AC²=27AC=3√3
