∵∠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°
解:设∠AOB=x,则∠COB=2x,∠DOC=∠DOA=3x
x+2x+3x+3x=360
x=40°
∴∠AOB=40°,∠DOC=120°