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Parallel and Perpendicular Lines
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Parallel Lines and Transversals

Theorem. Alternate Exterior Angles Theorem
If two parallel lines are cut by a transversal, then alternate exterior angles are congruent.

Refer to the given figure. If mn, then 27 and 18. Prove the congruence of the first angle pair and the second pair can be proven similarly.

Given: mn cut by transversal t
Prove: 27

Proof: