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If a transversal is perpendicular to two lines, the two lines are parallel.
Based on the diagram, the following statement holds true.
If t ⊥ l_1 and t ⊥ l_2, then l_1 ∥ l_2.
Angles ∠ 1 and ∠ 2 are corresponding angles and they are congruent. By the Converse Corresponding Angles Theorem, l_1 and l_2 are parallel.
This proves the theorem.
If t ⊥ l_1 and t ⊥ l_2, then l_1 ∥ l_2.