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Two lines with slopes m₁ and m₂ are PERPENDICULAR if

Am₁ − m₂ = 1
Bm₁ + m₂ = 0
Cm₁ = m₂
Dm₁ · m₂ = −1
Answer & Solution
Correct answer: D. m₁ · m₂ = −1
Perpendicular non-vertical lines satisfy m₁ · m₂ = −1.
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