延长BD到E,并使DE=DC,∠CDE=180°-∠BDC=
180°-120°=60°
△DCE为等边△→ DC=EC ------边
∠DCE =60°=∠ACB → ∠ACD =∠BCE----角
△ABC为等边△→AC=BC----------------边
边角边 →△ACD≌△BCE AD=BE=BD+DE=BD+DC