Angle Relationships with Parallel Lines

This video introduces the relationship between the angles formed by parallel lines and an intersecting line called a transversal.

When parallel lines intersect a transversal, corresponding angles are equal. These angles are taken to be equal without proof and the statement that corresponding angles are equal is a postulate (axiom) in geometry. From this statement and the fact that vertical angles are equal, it can be deduced that alternate interior and alternate exterior angles are also equal while same side interior angles are supplementary.

Khan Academy exercise to practice: