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Two lines with direction ratios $(a_1,b_1,c_1)$ and $(a_2,b_2,c_2)$ are perpendicular if:
A$a_1a_2+b_1b_2+c_1c_2=0$
B$a_1a_2+b_1b_2+c_1c_2=1$
C$\tfrac{a_1}{a_2}=\tfrac{b_1}{b_2}=\tfrac{c_1}{c_2}$
D$a_1+a_2=b_1+b_2$
Answer & Solution
Correct answer: A. $a_1a_2+b_1b_2+c_1c_2=0$
Lines are perpendicular when the sum of products of their direction ratios is zero: a₁a₂+b₁b₂+c₁c₂=0.
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