∠COD= ∠AOD- ∠AOC
= 120 -90 = 30°
∵ OC是∠BOD的平分线
∴ ∠BOD = ∠ COD = 30°
∴ ∠ AOB = ∠AOD +∠ BOD
=120+30 = 150°
∠AOB+∠COD = 180°