如圖,已知AB=AC,DB=DC,試說明∠ABD=∠ACD.

如圖,已知AB=AC,DB=DC,試說明∠ABD=∠ACD.

證明:∵AB=AC,∴∠ABC=∠ACB.∵BD=CD.∴∠DBC=∠DCB.∴∠ABC-∠DBC=∠ACB-∠DCB.即∠ABD=∠ACD.