∵ AB=AC∴ ∠ABC=∠ACB∵ ∠ABC+∠ACB+∠BAC=180∴ ∠BAC=180-2∠ACB= 2（90-∠ACB）∵ BD⊥AC∴ ∠BDC=90∵ ∠DBC+∠BDC+∠ACB=180∴ ∠DBC=180-∠BDC-∠ACB=180-90-∠ACB=90-∠ACB∴ ∠DBC=1/2∠BAC
