If a is a nonzero vector such that its projections on the vectors 2 i - j + 2 k, i + 2 j - 2 k and k are equal, then a unit vector along a is :
- 1 155(-7 i + 9 j + 5 k)
- 1 155(-7 i + 9 j - 5 k)
- 1 155(7 i + 9 j - 5 k)
- 1 155(7 i + 9 j + 5 k)
Answer
(4) 1 155(7 i + 9 j + 5 k)
Solution
Let a = x i + y j + z k. Projection on b = a b| b|; the magnitudes of the three vectors are 3, 3 and 1.
2x - y + 2z3 = x + 2y - 2z3 = z
From the first two: 2x - y + 2z = x + 2y - 2z x - 3y + 4z = 0 ...(i)
From the last two: x + 2y - 2z = 3z x + 2y - 5z = 0 ...(ii)
(ii) − (i): 5y - 9z = 0 y = 9z5, then x = 5z - 2y = 7z5
a 7 i + 9 j + 5 k, |7 i + 9 j + 5 k| = 49 + 81 + 25 = 155
Unit vector along a = 1 155(7 i + 9 j + 5 k) (the opposite unit vector is not among the options)
Answer: (4)