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Vector Algebra question

2023 · 30 Jan · Shift 2 · Q32
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  5. /2023 · 30 Jan · Shift 2 · Q32

Vector Algebra question

2023 · 30 Jan · Shift 2 · Q32

JEE MainMathematicsVector AlgebraMCQ+4 / −1
Let a⃗\vec{a}a and b⃗\vec{b}b be two vectors, Let ∣a⃗∣=1,∣b⃗∣=4|\vec{a}|=1,|\vec{b}|=4∣a∣=1,∣b∣=4 and a⃗⋅b⃗=2\vec{a} \cdot \vec{b}=2a⋅b=2. If c⃗=(2a⃗×b⃗)−3b⃗\vec{c}=(2 \vec{a} \times \vec{b})-3 \vec{b}c=(2a×b)−3b, then the value of b⃗⋅c⃗\vec{b} \cdot \vec{c}b⋅c is :
  1. A
    −48-48−48
  2. B
    −60-60−60
  3. C
    −84-84−84
  4. D
    −24-24−24
View written solutionFree

Correct answer: A

  1. We are given: ∣a⃗∣=1,∣b⃗∣=4,a⃗⋅b⃗=2|\vec a|=1,\quad |\vec b|=4,\quad \vec a\cdot \vec b=2∣a∣=1,∣b∣=4,a⋅b=2 and c⃗=(2 a⃗×b⃗)−3b⃗\vec c=(2\,\vec a\times \vec b)-3\vec bc=(2a×b)−3b

  2. We need to find: b⃗⋅c⃗\vec b\cdot \vec cb⋅c

  3. Substitute the value of c⃗\vec cc: b⃗⋅c⃗=b⃗⋅((2a⃗×b⃗)−3b⃗)\vec b\cdot \vec c=\vec b\cdot \left((2\vec a\times \vec b)-3\vec b\right)b⋅c=b⋅((2a×b)−3b)

  4. Use distributive property of dot product: b⃗⋅c⃗=2 b⃗⋅(a⃗×b⃗)−3(b⃗⋅b⃗)\vec b\cdot \vec c=2\,\vec b\cdot (\vec a\times \vec b)-3(\vec b\cdot \vec b)b⋅c=2b⋅(a×b)−3(b⋅b)

  5. Now, b⃗⋅(a⃗×b⃗)=0\vec b\cdot (\vec a\times \vec b)=0b⋅(a×b)=0 because a⃗×b⃗\vec a\times \vec ba×b is perpendicular to b⃗\vec bb.

    Also, b⃗⋅b⃗=∣b⃗∣2=42=16\vec b\cdot \vec b=|\vec b|^2=4^2=16b⋅b=∣b∣2=42=16

  6. Therefore, b⃗⋅c⃗=2(0)−3(16)=−48\vec b\cdot \vec c=2(0)-3(16)=-48b⋅c=2(0)−3(16)=−48

  7. Hence the correct option is: −48\boxed{-48}−48​ which is Option A.

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