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Vector Algebra question

2007 · Shift 0 · Q54
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  5. /2007 · Shift 0 · Q54

Vector Algebra question

2007 · Shift 0 · Q54

JEE MainMathematicsVector AlgebraMCQ+4 / −1
If u^\widehat uu and v^\widehat vv are unit vectors and θ\thetaθ is the acute angle between them, then 2u^×3v^2\widehat u \times 3\widehat v2u×3v is a unit vector for :
  1. A
    no value of θ\thetaθ
  2. B
    exactly one value of θ\thetaθ
  3. C
    exactly two values of θ\thetaθ
  4. D
    more than two values of θ\thetaθ
View written solutionFree

Correct answer: B

  1. Let us simplify the given vector:

2u^×3v^=6(u^×v^)2\widehat u \times 3\widehat v = 6(\widehat u \times \widehat v)2u×3v=6(u×v)

We are given that u^\widehat uu and v^\widehat vv are unit vectors, and the angle between them is θ\thetaθ.

  1. Use the magnitude formula for cross product:

∣u^×v^∣=∣u^∣ ∣v^∣sin⁡θ|\widehat u \times \widehat v| = |\widehat u|\,|\widehat v|\sin\theta∣u×v∣=∣u∣∣v∣sinθ

Since both are unit vectors,

∣u^∣=∣v^∣=1|\widehat u|=|\widehat v|=1∣u∣=∣v∣=1

so,

∣u^×v^∣=sin⁡θ|\widehat u \times \widehat v| = \sin\theta∣u×v∣=sinθ

Therefore,

∣2u^×3v^∣=∣6(u^×v^)∣=6sin⁡θ|2\widehat u \times 3\widehat v| = |6(\widehat u \times \widehat v)| = 6\sin\theta∣2u×3v∣=∣6(u×v)∣=6sinθ

  1. For 2u^×3v^2\widehat u \times 3\widehat v2u×3v to be a unit vector, its magnitude must be 111:

6sin⁡θ=16\sin\theta = 16sinθ=1

sin⁡θ=16\sin\theta = \frac{1}{6}sinθ=61​

  1. Now θ\thetaθ is acute, so

0<θ<π20 < \theta < \frac{\pi}{2}0<θ<2π​

In the acute interval, sin⁡θ\sin\thetasinθ is strictly increasing, so the equation

sin⁡θ=16\sin\theta = \frac{1}{6}sinθ=61​

has exactly one solution.

  1. Hence, there is exactly one value of θ\thetaθ.

Therefore, the correct option is:

B\boxed{\text{B}}B​

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