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JEE Advanced 2023 Paper 1 · Mathematics · Straight Lines and Circles · hard

Vec r=(1,0,-1)+λ(1,2,2) and L2: vec r=(2,3,0)+μ(2,-2,1).

One or more options correct

Consider the lines L1: r⃗=(1,0,−1)+λ(1,2,2)L_1:\ \vec r=(1,0,-1)+\lambda(1,2,2) and L2: r⃗=(2,3,0)+μ(2,−2,1)L_2:\ \vec r=(2,3,0)+\mu(2,-2,1). Which of the following is/are correct?
  1. A. L1L_1 and L2L_2 are skew lines and the shortest distance between them is 11
  2. B. The acute angle between L1L_1 and L2L_2 is cos⁡−1 ⁣(49)\cos^{-1}\!\left(\dfrac49\right)
  3. C. The plane containing L1L_1 and parallel to L2L_2 is 2x+y−2z=22x+y-2z=2
  4. D. The foot of the common perpendicular on L2L_2 is (83,73,23)\left(\dfrac83,\dfrac73,\dfrac23\right)
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Answer: A

Directions: d⃗1=(1,2,2)\vec d_1=(1,2,2), d⃗2=(2,−2,1)\vec d_2=(2,-2,1), d⃗1⋅d⃗2=0\vec d_1\cdot\vec d_2=0, so the lines are perpendicular. d⃗1×d⃗2=(6,3,−6)=3(2,1,−2)\vec d_1\times\vec d_2=(6,3,-6)=3(2,1,-2), ∣⋅∣=9|{\cdot}|=9. With AB⃗=(1,3,1)\vec{AB}=(1,3,1), AB⃗⋅(6,3,−6)=9\vec{AB}\cdot(6,3,-6)=9, so the shortest distance is 9/9=1≠09/9=1\neq0: skew lines.
(A) True: distance 11, and not intersecting.
(B) False: the angle is 90∘90^\circ, not cos⁡−1(4/9)\cos^{-1}(4/9).
(C) False: the plane through L1L_1 parallel to L2L_2 has normal (2,1,−2)(2,1,-2) and passes through (1,0,−1)(1,0,-1): 2x+y−2z=42x+y-2z=4, not 22.
(D) False: solving for the common perpendicular gives λ=1, μ=13\lambda=1,\ \mu=\frac13; the feet are (2,2,1)(2,2,1) on L1L_1 and (83,73,13)\left(\frac83,\frac73,\frac13\right) on L2L_2, so the stated zz-coordinate 23\frac23 is wrong.
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Original question written in the JEE Advanced 2023 Paper 1 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake