设 ∠ B = ∠ E , ∠ A C B = ∠ F , B C = E F \angle B=\angle E,\angle ACB=\angle F,BC=EF ∠B=∠E,∠ACB=∠F,BC=EF
若 A B ≠ D E AB\ne DE AB=DE
设 G B = D E GB=DE GB=DE， △ G B C ≅ △ D E F \triangle GBC\cong\triangle DEF △GBC≅△DEF
则 ∠ G C B = ∠ F = ∠ A C B \angle GCB=\angle F=\angle ACB ∠GCB=∠F=∠ACB，矛盾
则 A B = D E AB=DE AB=DE
∵ B C = E F , ∠ B = ∠ E \because BC=EF,\angle B=\angle E ∵BC=EF,∠B=∠E
∴ △ A B C ≅ △ D E F \therefore \triangle ABC\cong\triangle DEF ∴△ABC≅△DEF
∴ A B = D E , A C = D F , ∠ A = ∠ D \therefore AB=DE,AC=DF,\angle A=\angle D ∴AB=DE,AC=DF,∠A=∠D
