∵BD平分∠ABC，∴∠ABD=∠CBD，∵AD∥BC，∴∠ADB=∠DBC，∴∠ADB=∠ABD，∴AD=AB=CD．∵∠ABC=∠DCB，∴∠C=2∠ABD．∵BD⊥CD，∴∠DBC=30°，∴BC=2CD．设腰长是xcm，则AD=AB=CD=xcm，BC=2xcm，∵梯形ABCD的周长...