估计想求:∠A的度数.设∠C的外角为∠ACE;设∠ABD=∠DBC=X(度),∠ACD=∠ECD=Y(度).∠ACE=∠A+∠ABC,即2Y=∠A+2X,则2X=2Y-∠A.--------------------(1)∠D+∠CBD=∠DCE,即∠D+X=Y,则2∠D+2X=2Y;-------------(2)所以,2...