Which angles measure 55? How do you know?
How do you find angles congruent to the given
Look for angles that are corresponding and vertical to the given angle.
You can use the Corresponding Angles Postulate to prove other angle relationships.
Theorem
If a transversal intersects two parallel lines, then alternate interior angles are congruent.
If …
Then …
Theorem
If a transversal intersects two parallel lines, then same-side interior angles are supplementary.
If …
Then …
You will prove Theorem 3-2 in Exercise 25.