在DC上取一点E,使得：DE = BD .
则AD是BE的垂直平分线,可得：AB = AE ,∠B = ∠AEB .
因为,CE = DC-DE = DC-BD = AB = AE,
所以,∠EAB = ∠C ,
可得：∠B = ∠AEB = ∠EAB+∠C = 2∠C .