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Vector Algebra · Question 34

JEE Main 2025 (Online) 2nd April Morning ShiftMCQMedium📈 Topic in 42% papers+4−1
1

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. 1 155(-7 i + 9 j + 5 k)
  2. 1 155(-7 i + 9 j - 5 k)
  3. 1 155(7 i + 9 j - 5 k)
  4. 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)

JEE Main 2025 (Online) 2nd April Morning Shift

Keep practising — Vector Algebra

2

Let a = 2 i - 3 j + k, b = 3 i + 2 j + 5 k and a vector c be such that ( a - c) b = -18 i - 3 j + 12 k and a c = 3. If b c = d, then | a d| is equal to :

  1. 15
  2. 18
  3. 12
  4. 9
JEE Main 2025 (Online) 2nd April Evening Shift
3

Let a = 2 i - j + 3 k, b = 3 i - 5 j + k and c be a vector such that a c = c b and ( a + c) ( b + c) = 168. Then the maximum value of | c|2 is :

  1. 77
  2. 154
  3. 308
  4. 462
JEE Main 2025 (Online) 29th January Morning Shift
4

Let a = i + j + k, b = 2 i + 2 j + k and d = a b. If c is a vector such that a c = | c|, | c - 2 a|2 = 8 and the angle between d and c is 4, then |10 - 3 b c| + | d c|2 is equal to .

JEE Main 2025 (Online) 28th January Morning Shift
5

If the components of a = i + j + k along and perpendicular to b = 3 i + j - k respectively, are 1611(3 i + j - k) and 111(-4 i - 5 j - 17 k), then 2 + 2 + 2 is equal to:

  1. 16
  2. 23
  3. 26
  4. 18
JEE Main 2025 (Online) 28th January Evening Shift
6

Let A, B, C be three points in xy-plane, whose position vectors are given by 3 i + j, i + 3 j and a i + (1 - a) j respectively with respect to the origin O. If the distance of the point C from the line bisecting the angle between the vectors OA and OB is 9 2, then the sum of all the possible values of a is:

  1. 2
  2. 0
  3. 92
  4. 1
JEE Main 2025 (Online) 28th January Evening Shift

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