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JEE Advanced 2018 Paper 1 · Mathematics · Three Dimensional Geometry · medium

Let L1: (x-1)/(1)=(y-2)/(2)=(z-3)/(2) and L2: (x-3)/(2)=(y-1)/(1)=(z)/(-2).

One or more options correct

Let L1: x−11=y−22=z−32L_1:\ \dfrac{x-1}{1}=\dfrac{y-2}{2}=\dfrac{z-3}{2} and L2: x−32=y−11=z−2L_2:\ \dfrac{x-3}{2}=\dfrac{y-1}{1}=\dfrac{z}{-2}. Which of the following is (are) correct?
  1. A. L1L_1 and L2L_2 are skew lines
  2. B. The shortest distance between L1L_1 and L2L_2 is 11
  3. C. The acute angle between L1L_1 and L2L_2 is 60∘60^\circ
  4. D. The plane containing L1L_1 and parallel to L2L_2 has equation 2x−2y+z=12x-2y+z=1
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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). Their dot product is 2+2−4=02+2-4=0 (so the lines are perpendicular, not at 60∘60^\circ; C false) and d⃗1×d⃗2=(−6,6,−3)=−3(2,−2,1)\vec d_1\times\vec d_2=(-6,6,-3)=-3(2,-2,1). Take n⃗=(2,−2,1)\vec n=(2,-2,1), ∣n⃗∣=3|\vec n|=3, and AB→=(2,−1,−3)\overrightarrow{AB}=(2,-1,-3) between the points A(1,2,3)∈L1A(1,2,3)\in L_1, B(3,1,0)∈L2B(3,1,0)\in L_2. Then AB→⋅n⃗=4+2−3=3\overrightarrow{AB}\cdot\vec n=4+2-3=3, so the shortest distance is 33=1≠0\frac33=1\neq0: the lines are skew (A true) and the distance is 11 (B true). The plane through AA with normal n⃗\vec n contains L1L_1 and is parallel to L2L_2: 2x−2y+z=2−4+3=12x-2y+z=2-4+3=1 (D true).
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