use informal arguments to establish facts about the angle sum and exterior angle of triangles, the angles created when parallel lines are cut by a transversal, and the angle- angle criterion for similarity of triangles

What is the Angle-Angle Postulate for Triangle Similarity?
How Do You Find a Value for x that Makes Two Lines Parallel?

Got a transversal intersecting two lines? Trying to figure out if those lines are parallel? You could test to see if corresponding angles are congruent. This tutorial shows you how!
What is a Transversal?

Ever heard of a transversal? It's not as confusing as the term sounds. This tutorial will introduce you to transversals and show you the neat things that happen when a transversal meets two parallel lines. Take a look!
What are Interior and Exterior Angles of a Polygon?

Sometimes a math term can really tell you a lot about the thing it's describing. Think about the terms interior angle and exterior angle. Can you guess where each is located on a polygon? Take a look at this tutorial to find the answer and learn about interior and exterior angles!
What is the Triangle Sum Theorem?

When you're dealing with triangles, the Triangle Sum theorem can be very useful in finding interior angle measurements. In this tutorial, learn how to find this helpful theorem!