证明：过点C作CE‖AD交BA的延长线于点E,
∵CE‖AD,∴∠BAD=∠E,∠DAC=∠ACE,
∵AD为角平分线,∴∠BAD=∠DAC,∴∠E=∠ACE,∴AE=AC,
∵CE‖AD,∴BA/AE=BD/DC,∴AB/AC=BD/DC