## Geometry: Common Core (15th Edition)

The triangles are connected by a pair of vertical angles, which are congruent; therefore, we have one angle congruent in the two triangles. If $CD$ and $AB$ are parallel, alternate interior angles are congruent. This makes $\angle D ≅ \angle A$. So, we have two consecutive angles congruent in both triangles; therefore, the triangles are similar according to the Angle-Angle (AA) Similarity Postulate.