过E在平面β内作DE⊥BC于E,再在面ABC内,过E作EF⊥AB于F,连结DF因α⊥β,α∩β=l,AC在α内,AC⊥l故AC⊥β又DE在β内故AC⊥DE又DE⊥BC故DE⊥面ABC又EF,AB在面ABC内故DE⊥EF,DE⊥AB又EF⊥AB故AB⊥面DEF故AB⊥DF即∠DFE...