∵∠ADC是△ABD的外角
∴∠ADC＝∠ABC+∠BAD
∵∠BAD＝∠ABC
∴∠ADC＝2∠BAD
∵∠ADC＝∠ACD
∴∠DAC＝180-∠ACD-∠ADC＝180-2∠ADC＝180-4∠BAD
∵∠BAC＝∠BAD+∠DAC,∠BAC＝78
∴∠BAD+∠DAC＝78
∴∠DAC＝78-∠BAD
∴180-4∠BAD＝78-∠BAD
∴∠BAD＝34
∴∠DAC＝78-34＝44°