过点C作CE‖AD交BA的延长线于E,
∴∠BAD=∠E,∠DAC=∠ACE,
∵∠BAD=∠DAC,
∴∠E=∠ACE,
∴AE=AC
∵CE‖AD,
∴BA：AE=BD：DC,
∴AB：AC=BD：DC