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Lines with direction ratios (a1, b1, c1) and (a2, b2, c2) are perpendicular when:
Aa1 + a2 = b1 + b2 = c1 + c2
Ba1a2 + b1b2 + c1c2 = 1
Ca1/a2 = b1/b2 = c1/c2
Da1a2 + b1b2 + c1c2 = 0
Answer & Solution
Correct answer: D. a1a2 + b1b2 + c1c2 = 0
Perpendicular means cos θ = 0, which happens when the dot product of the ratios vanishes.
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