延长AC到E使CE=DC,连DE则:AC+DC=AC+CE=AE∠E=∠CDE而角C=2角B所以∠B=∠EAD是角BAC的角平分线,∠EAD=∠BAD∠ADE=∠ADC+∠CDE=(∠DAB+∠B)+∠CDE=(∠DAE+∠E)+∠CDE=∠DAE+(∠E+∠CDE)=∠DAE+∠DCA=∠ADBAD=AD所以△A...