Let (α,β,γ) be the co-ordinates of the foot of the perpendicular drawn from the point (5,4,2) on the line r=(−i^+3j^+k^)+λ(2i^+3j^−k^). Then the length of the projection of the vector αi^+βj^+γk^ on the vector 6i^+2j^+3k^ is :
(A)715
(B)4
(C)718✓ Correct
(D)3
Answer: (C)
Solution
A general point of the line is (2λ−1,3λ+3,1−λ), and the vector from (5,4,2) to it is perpendicular to the direction (2,3,−1) when 14λ−14=0, i.e. λ=1. The foot of the perpendicular is (α,β,γ)=(1,6,0). The length of the projection of i^+6j^ on 6i^+2j^+3k^ is 36+4+9∣6+12+0∣=718.
JEE Main · 2026 · 21 Jan 2026 · Shift 1 · Single correct option