∵∠AOB=½∠BOC,,∠COD=∠AOD=3∠AOB∠AOB+∠BOC+∠COD+∠AOD=360°所以2∠AOB=∠BOC,把其他三个角全用∠AOB代替有∠AOB+∠BOC+∠COD+∠AOD=360°∠AOB+2∠AOB+3∠AOB+3∠AOB=360°9∠AOB=360°∠AOB=40°∴∠COD=∠AOD=3∠AOB=120°