If two straight lines whose direction cosines are given by the relations l + m − n = 0, 3l^ 2 + m^ 2 + cnl = 0, are parallel, then the positive value of c is :

JEE Main 2022 Mathematics question

If two straight lines whose direction cosines are given by the relations l + m − n = 0, + cnl = 0, are parallel, then the positive value of c is :

Answer: (1)

From l + m − n = 0, n = l + m. Substituting in + cnl = 0: , i.e. lm + , or + c(l/m)

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Question ID VK-034609