若OB在∠AOC内,则 ∠DOE = ∠COE-∠COD = (1/2)(∠AOC-∠BOC) = (1/2)∠AOB = 40° ；
若OA在∠BOC内,则 ∠DOE = ∠COD-∠COE = (1/2)(∠BOC-∠AOC) = (1/2)∠AOB = 40° ；
综上可得：∠DOE = 40° .