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Direction cosines of 2 lines are given by the equations 3l+m+5n=0,6mn−2nl+5lm=03l+m+5n=0, 6mn-2nl+5lm=0 then which of the following statements are correct ?

(A) Direction ratios of two lines are (1, 2, -1) and (-2, 1, 1).

(B) Angle between lines is π3\frac{\pi}{3}.

(C) The angle between the lines is cos⁡−1(−16)\cos^{-1}\left(-\frac{1}{6}\right).

(D) Vectors parallel to these lines are i^+2j^−k^\hat i+2\hat j-\hat k and −2i^+j^+k^-2\hat i+\hat j+\hat k.

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 4

Substituting m=−3l−5nm=-3l-5n into the second equation reduces it to l2+3ln+2n2=0l^2+3ln+2n^2=0, i.e. (l+n)(l+2n)=0(l+n)(l+2n)=0.

This gives direction ratios (1,2,−1)(1,2,-1) and (−2,1,1)(-2,1,1), so (A) and (D) hold.

Then cos⁡θ=−2+2−166=−16\cos\theta=\frac{-2+2-1}{\sqrt6\sqrt6}=-\frac{1}{6}, making (C) true and (B) false.

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