$ 解:∠BOD=180°-∠AOB=60°$ $∵OC平分∠BOD,∴∠COB=∠COD=\frac {1}{2}∠BOD=30°$ $解:∵OB平分∠AOD,OC平分∠BOD$ $∴∠AOB=∠DOB,∠BOC=∠DOC$ $∴∠AOD=2∠BOD=4∠BOC$ $解:平分,理由:$ $易求,∠COB=\frac {2}{2+1}∠AOB=80°,∠AOC=\frac {1}{1+2}∠AOB=40°$ $∴∠AOD=∠AOC+∠COD=60°$ $∴∠BOD=∠BOC-∠COD=60°$ $∴∠AOD=∠BOD,即OD平分∠AOB$
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