∠∠ ADB =∠CDE, DB=DC

∴δadb≌δedc

∴∠E=∠ 1

Do δ a' e' c, AE = a' e', AC = a' c' and CE = c' e' in the same way.

∴δaec≌δa'e'c'

∴∠3=∠4

∠∠2 =∠e ' =∠e =∠∠ 1。

∴∠BAC=∠B'A'C'

ab = a' b '，AC = a' c '

∴∠abc≌δa'b'c'