∵OM,ON分别平分∠AOC,∠BOC,
∴若∠MOC=1/2∠AOC,∠CON=1/2∠BOC,
∴∠MON=∠MOC+∠CON
=1/2∠AOC+1/2∠BOC
=1/2(∠AOC+∠BOC)
=1/2∠AOB=0.5×80°=40°