在平行四边形ABCD中,AB = CD ,∠BAC = ∠DCA ；因为,在△ABE和△CDF中,AB = CD ,∠BAE = ∠DCF ,AE = CF ,所以,△ABE ≌ △CDF ,可得：BE = DF ,∠ABE = ∠CDF ；则有：∠BEF = ∠BAE+∠ABE = ∠DCF+∠CDF = ∠DFE ,...