Let A and B be two distinct points on the line L : x - 63 = y - 72 = z - 7-2. Both A and B are at a distance 2 17 from the foot of perpendicular drawn from the point (1, 2, 3) on the line L. If O is the origin, then OA OB is equal to
- 49
- 21
- 47
- 62
Answer
(3) 47
Solution
Any point of L: (6 + 3t, 7 + 2t, 7 - 2t); direction ratios of L: (3, 2, -2).
Let F be the foot of the perpendicular from (1, 2, 3). Then
(5 + 3t, 5 + 2t, 4 - 2t) (3, 2, -2) = 0
15 + 9t + 10 + 4t - 8 + 4t = 0
17 + 17t = 0 t = -1
F = (3, 5, 9)
Unit vector along L: (3, 2, -2) 17, so points at distance 2 17 from F are
F 2(3, 2, -2)
A = (9, 9, 5), B = (-3, 1, 13)
OA OB = 9(-3) + 9(1) + 5(13)
= -27 + 9 + 65 = 47
Answer: (3)