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Vector Algebra · Question 11

JEE Main 2026 (Online) 28th January Morning ShiftMCQEasy📈 Topic in 42% papers+4−1
1

For three unit vectors a, b, c satisfying | a - b|2 + | b - c|2 + | c - a|2 = 9 and |2 a + k b + k c| = 3, the positive value of k is :

  1. 4
  2. 5
  3. 6
  4. 3

Answer

(2) 5

Solution

| a - b|2 + | b - c|2 + | c - a|2 = 6 - 2( a b + b c + c a) = 9

a b + b c + c a = - 32

| a + b + c|2 = 3 + 2 (- 32 ) = 0

a + b + c = 0 b + c = - a

|2 a + k( b + c)| = |(2 - k) a| = |2 - k| = 3

k = 5 or k = -1

Positive value: k = 5

Answer: (2)

JEE Main 2026 (Online) 28th January Morning Shift

Keep practising — Vector Algebra

2

Let PQR be a triangle such that PQ = -2 i - j + 2 k and PR = a i + b j - 4 k, a, b Z. Let S be the point on QR, which is equidistant from the lines PQ and PR. If | PR| = 9 and PS is parallel to i - 7 j + 2 k, then the value of 3a - 4b is .

JEE Main 2026 (Online) 28th January Morning Shift
3

Let P be a point in the plane of the vectors AB = 3 i + j - k and AC = i - j + 3 k such that P is equidistant from the lines AB and AC. If | AP| = 52, then the area of the triangle ABP is :

  1. 32
  2. 304
  3. 264
  4. 2
JEE Main 2026 (Online) 28th January Evening Shift
4

Let a = 2 i + j - 2 k, b = i + j and c = a b. Let d be a vector such that | d - a| = 11, | c d| = 3 and the angle between c and d is 4. Then a d is equal to

  1. 0
  2. 1
  3. 3
  4. 11
JEE Main 2026 (Online) 24th January Morning Shift
5

Let a = 2 i - j - k, b = i + 3 j - k and c = 2 i + j + 3 k. Let v = a + b, R, be the vector in the plane of the vectors a and b, such that the length of its projection on the vector c is 1 14. Then | v| is equal to

  1. 352
  2. 13
  3. 212
  4. 7
JEE Main 2026 (Online) 24th January Evening Shift
6

Let a = 2 i - 5 j + 5 k and b = i - j + 3 k. If c is a vector such that 2( a c) + 3( b c) = 0 and ( a - b) c = -97, then | c k|2 is equal to

  1. 193
  2. 205
  3. 218
  4. 233
JEE Main 2026 (Online) 24th January Evening Shift
7

Let a = - i + j + 2 k, b = i - j - 3 k, c = a b and d = c a. Then ( a - b ) d is equal to :

  1. 2
  2. -2
  3. 4
  4. -4
JEE Main 2026 (Online) 23rd January Morning Shift
8

Let a, b, c be three vectors such that a b = 2( a c). If | a| = 1, | b| = 4, | c| = 2, and the angle between b and c is 60 , then | a c| is equal to

  1. 2
  2. 0
  3. 1
  4. 4
JEE Main 2026 (Online) 23rd January Evening Shift
9

Let a = i - 2 j + 3 k, b = 2 i + j - k, c = i + j + k and v = a b. If v c = 11 and the length of the projection of b on c is p, then 9p2 is equal to

  1. 9
  2. 6
  3. 4
  4. 12
JEE Main 2026 (Online) 23rd January Evening Shift
10

Let AB = 2 i + 4 j - 5 k and AD = i + 2 j + k, R. Let the projection of the vector v = i + j + k on the diagonal AC of the parallelogram ABCD be of length one unit. If , , where > , be the roots of the equation 2x2 - 6 x + 5 = 0, then 2 - is equal to

  1. 3
  2. 6
  3. 4
  4. 1
JEE Main 2026 (Online) 22nd January Morning Shift
11

Let a = 2 i - j + k and b = j + 2 k, Z be two vectors. Let c = a b and d be a vector of magnitude 2 in yz-plane. If | c| = 53, then the maximum possible value of ( c d)2 is equal to :

  1. 52
  2. 26
  3. 104
  4. 208
JEE Main 2026 (Online) 22nd January Evening Shift

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