The point P is the intersection of the straight line joining the points Q(2,3,5) and R(1,−1,4) with the plane 5x−4y−z=1. If S is the foot of the perpendicular drawn from the point T(2,1,4) to QR, then the length of the line segment PS is
(A)21✓ Correct
(B)2
(C)2
(D)22
Answer: (A)
Solution
The line QR has direction ratios (1,4,1) and parametric form (2−t,3−4t,5−t). Substituting into the plane equation: 5(2−t)−4(3−4t)−(5−t)=1 gives t=2/3, so P=(4/3,1/3,13/3). The vector PT=(2/3,2/3,−1/3) has magnitude ∣PT∣=1. The angle between QR and PT is 45° (from their dot product). Since S is the foot of perpendicular from T to QR, we have PS=PTsin45°=21.
JEE Advanced · 2012 · 08 Apr 2012 · Paper 1 · Single correct option