This is an applet that will help you investigate why SAS Similarity is valid.

Given construction: ∠B≅∠D and the sides that include this angle are proportional.
Explore: Move points A, B, or C.
How does this affect the remaining angles and ratio of the 3rd side?
Using the given construction, are these two triangles similar?
Explore: Change the scale factor.
How does this affect the remaining angles and ratio of the 3rd side?
Using the given construction, are these two triangles similar?