∵∠AOC＝∠AOB+∠BOC,ON平分∠AOC
∴∠CON＝∠AOC/2＝（∠AOB+∠BOC）/2
∵OM平分∠BOC
∴∠COM＝∠BOC/2
∴∠MON＝∠CON-∠COM＝（∠AOB+∠BOC）/2-∠BOC/2＝∠AOB/2＝90/2＝45°