Two lines with slopes m₁ and m₂ are PARALLEL if and only if
Am₁ · m₂ = −1
Bm₁ = m₂
Cm₁ + m₂ = 0
Dm₁ = 1/m₂
Answer & Solution
Correct answer: B. m₁ = m₂
Parallel ⇔ equal slopes (m₁ = m₂). Perpendicular ⇔ product −1.
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