因为 ∠DAB+∠DBA+∠ADB=180° ∠A+∠B+∠C=180°
所以 ∠DAB+∠DBA+∠ADB=∠A+∠B+∠C
∠DAB+∠DBA+∠ADB=∠DAB+∠DAC+∠DBA+∠DBC+∠C
所以 ∠ADB=∠DBC+∠DBC+∠C