AB = AC，AE=AD，∠B=∠C

∴△ABE≌△ACD,BE=CD

∴BE-DE=CD-DE, which means BD=CE.

7. Solution: ∫∠A = 40, AB = AC.

∴∠ABC=∠ACB=( 180 -40 )÷2=70

∫MN vertical division AB

∴DB=DA

∴∠A=∠ABD=40

∴∠DBC=∠ABC-∠ABD=70 -40 =30