Alternate Exterior Angles
As with alternate interior angles, we can replace one of two corresponding angles with its vertical angle. This gives us a pair of alternate exterior angles.
Again the word "alternate" means they are on opposite sides of the transversal, and "exterior" means the are not between the parallel lines (they are "outside").
Two lines are parallel if and only if alternate exterior angles are congruent.
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Date last modified: September 16, 2008.