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Let L1L_1 and L2L_2 be any two lines given by

L1:3i^+2j^−4k^+λ(i^+2j^+2k^)L_1: 3\hat{i}+2\hat{j}-4\hat{k} + \lambda(\hat{i}+2\hat{j}+2\hat{k})

L2:5i^−2j^+μ(3i^+2j^+6k^)L_2: 5\hat{i}-2\hat{j} + \mu(3\hat{i}+2\hat{j}+6\hat{k}) Where λ\lambda and μ\mu are parameters

then which of the following statements are TRUE?

A. Equation of a line passing through the point (1,2,3)(1,2,3) which is parallel to line L1L_1 is x−11=y−22=z−32\dfrac{x-1}{1} = \dfrac{y-2}{2} = \dfrac{z-3}{2}

B. Direction cosines of line L2L_2 are 37,27,67\dfrac{3}{7}, \dfrac{2}{7}, \dfrac{6}{7}

C. Line L1L_1 is parallel to the line L2L_2

D. The angle between the two lines L1L_1 and L2L_2 is cos⁡−1(1921)\cos^{-1}\left(\dfrac{19}{21}\right)

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 4

L1L_1 has direction (1,2,2)(1,2,2) and L2L_2 has (3,2,6)(3,2,6); these are not proportional, so C is false.

A is the standard symmetric form through (1,2,3)(1,2,3) with direction (1,2,2)(1,2,2), so it is true.

Since ∣(3,2,6)∣=7|(3,2,6)| = 7, B is true. Also cos⁡θ=(3+4+12)/(3×7)=19/21\cos\theta = (3+4+12)/(3\times7) = 19/21, so D is true.

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