ASA (Angle-Side-Angle) Postulate
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, 
.
Therefore, by the ASA postulate, 
.
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, .
Therefore, by the ASA postulate, .