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Vector Algebra · Question 36

JEE Main 2025 (Online) 29th January Morning ShiftMCQMedium📈 Topic in 52% papers+4−1
1

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

Answer

(3) 308

Solution

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

( a + b) c = 0

c = t( a + b) = t(5 i - 6 j + 4 k)

a b = 6 + 5 + 3 = 14

a c = t(10 + 6 + 12) = 28t, b c = t(15 + 30 + 4) = 49t

| c|2 = t2(25 + 36 + 16) = 77t2

( a + c) ( b + c) = a b + a c + c b + | c|2 = 168

14 + 77t + 77t2 = 168

t2 + t - 2 = 0 t = 1 or t = -2

| c|2 = 77 or 77 4 = 308

maximum value of | c|2 = 308

Answer: (3)

JEE Main 2025 (Online) 29th January Morning Shift

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2

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
3

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
4

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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