You can tell that the triangles are similar because of the SAS (side-angle-side) theorem.
This is because you already know that two of the sides are congruent since <u>AB and CD were shown to be congruent in the image</u>, and <u>AC and CA are congruent because they're shared</u>. You know that there are similar angles because of the Alternate Interior Angles Theorem*: because of it, you know <u>∠ACD and ∠CAB are similar because the lines that their transversal intersects are parallel</u>.
*The Alternate Interior Angles Theorem states that if two lines are parallel, the alternate interior angles formed by a transversal are congruent.
