延长AD交BC于E
∵∠BAD+∠ABD=90°,∠ABD=∠DBE
∴∠DBE+∠BAD=90°
∵∠DBE+∠BED=90°
∴∠BAD=∠BED
∵∠BED=∠ACB+∠DAC(外角定义)
∴∠BAD=∠ACB+∠DAC