证明：在BC上取BE=BD,连接DE,因为BD是∠ABC的平分线,∠A=100 ,AB=AC,所以∠BDE=∠BED=80,∠EDC=∠BED-=∠ACB=80-40=40,所以DE=EC,在△EDC和△ABC中,∠EDC=∠ABC,∠ACB为公共角,所以△EDC∽△ABC,所以AB/DE=BC/CD,即A...