如图，∠AOD=∠DOC+______=∠AOB-______．
计算：=________．
若成立，则x满足_____________________．
当a=时，则___________．
计算：=_________________．
计算：___________．
=________．
成立，则x满足_____________________．
时，则
___________．
=_________________．
___________．
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