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
