If two angles and the included side of a triangle are congruent to two angles and the included side of another triangle, then the two triangles are congruent.
In the figure, ∠ B ≅ ∠ E , B C ¯ ≅ E F ¯ and ∠ C ≅ ∠ F .
Therefore, by the ASA postulate, Δ A B C ≅ Δ D E F .
