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证明，可能不是很简明

Posted by huicpc0860 at 2010-04-13 17:22:35 on Problem 1673 and last updated at 2010-04-13 23:39:43 
以B为圆心旋转EBJ，先向BC边转90度，则BC，BJ重合，ABE'共线，角OBM'等于90度；
以B为圆心旋转EBJ，先向BA边转90度，则BA，BE重合，ABJ'共线，角OBM''等于90度，且M'BM''共线；
由M是中点还有AM''和CM'相等以及角AM''B和角CM'B互补(由旋转前的状态可知），知ACM'M''平行四边形，角OB垂直M''M'，所以OB与AC垂直。
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