JEE Mains · Maths · STD 12 - 11. three dimension geometry
If two straight lines whose direction cosines are given by the relations \(l+m-n=0,3l^{2}+m^{2}+c n l =0\) are parallel, then the positive value of \(c\) is
- A \(6\)
- B \(4\)
- C \(3\)
- D \(2\)
Answer & Solution
Correct Answer
(A) \(6\)
Step-by-step Solution
Detailed explanation
\(1+ m - n =0\) \(3 l^{2}+ m ^{2}+ cl (1+ m )=0\) \(n =1+ m\) \(3 l ^{2}+ m ^{2}+ cl ^{2}+ clm =0\) \((3+ c ) l ^{2}+ clm + m ^{2}=0\) \((3+c)\left(\frac{l}{m}\right)^{2}+c\left(\frac{l}{m}\right)+1=0 \ldots \ldots(1)\) \(\because\) lies are parallel. Roots of \((1)\) must be…
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