Use the Slopes of Perpendicular Lines Theorem to form expressions of the slopes.
See solution.
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If lines a and b are both parallel to line c, we can, by the Slopes of Parallel Lines Theorem, say that m_a=m_c and m_b=m_c. By the Transitive Property of Equality we know that m_a=m_b. Therefore, by the Slopes of Parallel Lines Theorem, a ∥ b.
