Hard MCQ +4 / -1 PYQ · JEE Mains 2024

Let $\overrightarrow{\mathrm{a}}=6 \hat{i}+\hat{j}-\hat{k}$ and $\overrightarrow{\mathrm{b}}=\hat{i}+\hat{j}$. If $\overrightarrow{\mathrm{c}}$ is a is vector such that $$|\overrightarrow{\mathrm{c}}| \geq 6, \overrightarrow{\mathrm{a}} \cdot \overrightarrow{\mathrm{c}}=6|\overrightarrow{\mathrm{c}}|,|\overrightarrow{\mathrm{c}}-\overrightarrow{\mathrm{a}}|=2 \sqrt{2}$$ and the angle between $\vec{a} \times \vec{b}$ and $\vec{c}$ is $60^{\circ}$, then $|(\vec{a} \times \vec{b}) \times \vec{c}|$ is equal to:

  1. A $\frac{3}{2} \sqrt{6}$
  2. B $\frac{9}{2}(6-\sqrt{6})$
  3. C $\frac{9}{2}(6+\sqrt{6})$ Correct answer
  4. D $\frac{3}{2} \sqrt{3}$

Solution

<p>$$\begin{aligned} & |(\vec{a} \times \vec{b}) \times \vec{c}|=|\vec{a} \times \vec{b}||\vec{c}| \sin 60^{\circ} \\ & \left|\begin{array}{ccc} i & j & k \\ 6 & 1 & -1 \\ 1 & 1 & 0 \end{array}\right|=i(1)-j(1)+k(5) \\ & =i-j+5 k \\ & |\vec{a} \times \vec{b}|=\sqrt{1+1+25}=\sqrt{27} \\ & |\vec{c}-\vec{a}|=2 \sqrt{2} \\ & c^2+a^2-2 a c=8 \\ & c^2-12 c+30=0 \\ & c=\frac{12+\sqrt{24}}{2}=6+\sqrt{6} \\ & \Rightarrow|(\vec{a} \times \vec{b}) \times \vec{c}|=\sqrt{27} \times(6+\sqrt{6}) \times \frac{\sqrt{3}}{2} \\ & =\frac{a}{2}(6+\sqrt{6}) \\ \end{aligned}$$</p>

About this question

Subject: Mathematics · Chapter: Vector Algebra · Topic: Types of Vectors

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