Let L 1 and L 2 denotes the lines r → = i ^ + λ - i ^ + 2 j ^ + 2 k ^ ,   λ ∈ R…

Let L1 and L2 denotes the lines
r=i^+λ-i^+2j^+2k^,λR and r=μ2i^-j^+2k^,μR
respectively. If L3 is a line which is perpendicular to both L1 and L2 and cuts both of them, then which of the following options describe(s) L3 ?
  1. r=132i^+k^+t2i^+2j^-k^,tR
  2. r=294i^+j^+k^+t2i^+2j^-k^,tR
  3. r=292i^-j^+2k^,t2i^+2j^-k^,tR
  4. r=t2i^+2j^-k^,tR

Solution

L1:r=i^+λ-i^+2j^+2k^,  λR

Let a general point on line L1 be A1-λ, 2λ, 2λ

Also L2:r=μ2i^-j^+2k^ 
let a general point on line L2 be B2μ, -μ, 2μ
Then  AB=2μ+λ-1i^+-μ-2λ j^+2μ-2λk^
Now, AB is perpendicular to both L1 and L2
Hence, (a) AB-i^+2j^+2k^=0 ( If a and b are perpendicular then ab=0 )
(b) AB2i^-j^+2k^=0

Using 
(a) AB-i+2j+2k=0
-12μ+λ-1+-μ-2λ2+2μ-2λ2=0
-2μ-λ+1-2μ-4μ+4μ-4λ=0
9λ=1
λ=19  i
From (b) we have      AB2i^-j^+2k^=0
2μ+λ-12+-μ-2λ-1+2μ-2λ2=0
4μ+2λ-2+μ+2λ+4μ-4λ=0
9μ=2
μ=29  ii
Using value of λ and μA89,29,29 and B49,-29,49
AB=-292i^+2j^-k^
Now, using point A and AB
Equation of line =89i^+29j^+29k^+γ-292i^+2j^-k^
Option (B) :
r=294i^+j^+k^+t2i^+2j^-k^,t R
Using point B and AB
Option (C) :
Equation of line r=292i^-j^+2k^+t2i^+2j^-k^, tR
Using midpoint of A and B , 23,0,13
Option (A) :
r=132i^+k^+t2i^+2j^-k^,tR

Asked in: JEE Advanced 2019 (Paper 1)

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