Because when a transversal line cuts through parallel lines it creates vertical opposite equal angles.
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1. Alternate Interior Angles 2. Alternate Exterior Angles 3. Corresponding Angles 4. Same-Side Interior Angles 5. Same-Side Exterior Angles
If two lines are cut by a transversal to form pairs of congruent corresponding angles, congruent alternate interior angles, or congruent alternate exterior angles, then the lines are parallel.
Both alternate interior and alternate exterior angle pairs lie on opposite sides of the transversal.
Corresponding and alternate angles
A. Corresponding parts of similar triangles are similar.B. Alternate interior angles are supplementary.C. Alternate interior angles are congruent.D. Corresponding parts of congruent triangles are congruent