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JEE Main202125 Jul 2021Morning ShiftMathematicsVector AlgebraActual

Let the vectors 2 + a + b i ^ + a + 2 b + c j ^ - b + c k ^ , 1 + b i ^ + 2 b j ^ - b k ^ and 2 + b i ^ + 2 b j ^ + 1 - b k ^ , ∀ a , b , c ∈ R be co-planar. Then which of the following is true?

Options

  1. A2 b = a + c
  2. B3 c = a + b
  3. Ca = b + 2 c
  4. D2 a = b + c

Correct answer

A. 2 b = a + c

Step-by-step solution

If the vectors are co-planar, a + b + 2 a + 2 b + c - b - c b + 1 2 b - b b + 2 2 b 1 - b = 0 Now R 1 ↔ R 1 - R 2 ,   R 3 ↔ R 3 - R 2   So a + 1 a + c - c b + 1 2 b - b 1 0 1 = 0 ⇒ ( a + 1 ) 2 b - ( a + c ) ( 2 b + 1 ) - c ( - 2 b ) = 0 ⇒ 2 a b + 2 b - 2 a b - a - 2 b c - c + 2 b c = 0 ⇒ 2 b - a - c = 0 ⇒ 2 b = a + c

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