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JEE Advanced · Mathematics · 31. 3D Geometry

From a point Pλ, λ, λ, perpendiculars PQ and PR are drawn respectively on the lines y=x, z=1 and y= -x, z= -1 . If P is such that QPR is a right angle, then the possible value (s) of λ is (are)

  1. A 2
  2. B 1
  3. C -1
  4. D -2
Verified Solution

Answer & Solution

Correct Answer

(C) -1

Step-by-step Solution

Detailed explanation

Line 1 : x1=y1=z-10-r, Qr, r, 1
Line 2 : x1=y-1=z+10=k, Rk, -k, -1
PQ=λ-r i^+λ-r j^+λ-1 k^
And λ-r+λ-r=0 as PQ is to L1
2λ=2r λ=r
PR=λ-ki^+λ+k j^+λ+1 k^
And λ-k-λ-k=0 as PR is to L2
k=0
So PQPR
λ-r λ-k+λ-r λ+k+λ-1 λ+1=0
λ=1, -1
For λ=1 as point P and Q coincide
λ= -1.
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