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JEE Advanced 2017 Paper 2 · Mathematics · Three Dimensional Geometry · hard

Vec r=λ(hat i+2hat j+2hat k) and L2: vec r=(3hat i+3hat k)+μ(2hat i+hat j-2hat k), where λ,μ∈mathbb R.

One or more options correct

Consider the lines L1: r⃗=λ(i^+2j^+2k^)L_1:\ \vec r=\lambda(\hat i+2\hat j+2\hat k) and L2: r⃗=(3i^+3k^)+μ(2i^+j^−2k^)L_2:\ \vec r=(3\hat i+3\hat k)+\mu(2\hat i+\hat j-2\hat k), where λ,μ∈R\lambda,\mu\in\mathbb R. Which of the following is/are correct?
  1. A. L1L_1 and L2L_2 are perpendicular and they do not intersect
  2. B. The shortest distance between L1L_1 and L2L_2 is 33
  3. C. The foot of the common perpendicular on L1L_1 is (2,4,4)(2,4,4)
  4. D. The plane containing L1L_1 and parallel to L2L_2 is 2x−2y+z=02x-2y+z=0
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Answer: A, B, D

Directions are d⃗1=(1,2,2)\vec d_1=(1,2,2) and d⃗2=(2,1,−2)\vec d_2=(2,1,-2). Since d⃗1⋅d⃗2=2+2−4=0\vec d_1\cdot\vec d_2=2+2-4=0 the lines are perpendicular. The common normal direction is d⃗1×d⃗2=(−6,6,−3)∥(2,−2,1)\vec d_1\times\vec d_2=(-6,6,-3)\parallel(2,-2,1) with magnitude 99. The vector from (0,0,0)(0,0,0) to (3,0,3)(3,0,3) has dot product −27-27 with (−6,6,−3)(-6,6,-3), so the shortest distance is 27/9=3≠027/9=3\ne0; lines are skew, hence A and B are true. The plane through the origin with normal (2,−2,1)(2,-2,1) contains L1L_1 (2−4+2=02-4+2=0) and is parallel to L2L_2 (4−2−2=04-2-2=0), so D is true. Solving the perpendicularity conditions gives λ=1,μ=0\lambda=1,\mu=0: the feet are (1,2,2)(1,2,2) and (3,0,3)(3,0,3), so C, which claims (2,4,4)(2,4,4), is false.
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