Description: Watch Sal work through a basic congruence and similarity problem. Now, it might jump out at you, at least it looks like it immediately, that triangle, the small triangle on the left is going to be similar to this larger triangle on the right. All right, and I could keep, if I wanted to extend this as a line, I could keep going in both directions, but the bottom line, not to be punny about it, is that these two things are parallel. This angle right over here, if you view segment B E as a transversal of the two parallel lines, that right over there is going to be an alternate interior angle.*Teacher Advisor is 100% free.