2:1
延长AB至E,使BE=BD,连结ED,EC,则AE=AC,又AD是角平分线,△AED≌△ACD
∠AED=∠ACB
又BE=BD,∠AED=∠BDE,
所以∠ABD=∠AED+∠BDE=2∠AED=2∠ACB,即∠B=2∠C