AD=BD,得到∠DBA=∠DAB AD=CD,得到∠DCA=∠DAC 由三角形内角和为180°,得到∠DBA+∠DCA+∠BAC=180° ∠DBA+∠DCA+∠DAB+∠DAC=180° 2*(∠DAB+∠DAC)=180° ∠DAB+∠DAC=90°,得到∠BAC=90°.
