做辅助线由点D向边AC引交AC与E使AB=AEAD角平线所角BAD=CAD公共边AD所边角边三角形ABD全等ADE所角AED=角B应角B=2倍角C角AED外角所等于角C+角EDC所角EDC=角C所边ED=ECAE=AB所AC=AB+BD
