Medium INTEGER +4 / -1 PYQ · JEE Mains 2023

Let $\vec{a}=\hat{i}+2 \hat{j}+3 \hat{k}$ and $\vec{b}=\hat{i}+\hat{j}-\hat{k}$. If $\vec{c}$ is a vector such that $\vec{a} \cdot \vec{c}=11, \vec{b} \cdot(\vec{a} \times \vec{c})=27$ and $\vec{b} \cdot \vec{c}=-\sqrt{3}|\vec{b}|$, then $|\vec{a} \times \vec{c}|^{2}$ is equal to _________.

Answer (integer) 285

Solution

Given, <br/><br/>$$ \begin{aligned} & \vec{a}=\hat{i}+2 \hat{j}+3 \hat{k} \\\\ & \vec{b}=\hat{i}+\hat{j}-\hat{k} \\\\ & \vec{a} \cdot \vec{c}=11 \\\\ & \vec{b} \cdot(\vec{a} \times \vec{c})=27 \\\\ & \vec{b} \cdot \vec{c}=-\sqrt{3}|\vec{b}| \\\\ & (\vec{b} \times \vec{a}) \cdot \vec{c}=27 \end{aligned} $$ <br/><br/>$\text { Let } \vec{c}=c_1 \hat{i}+c_2 \hat{j}+c_3 \hat{k}$ <br/><br/>As $\vec{a} \cdot \vec{c}=11$ <br/><br/>$\therefore$ $c_1+2 c_2+3 c_3=11$ ......(i) <br/><br/>Also, $\vec{b} \cdot \vec{c}=-\sqrt{3}|\vec{b}|$ <br/><br/>$$ \begin{aligned} & \therefore c_1+c_2-c_3=-\sqrt{3} \sqrt{3} \\\\ & \Rightarrow c_1+c_2-c_3=-3 ......(ii) \end{aligned} $$ <br/><br/>Also, $\vec{b} \cdot(\vec{a} \times \vec{c})=27$ <br/><br/>$\therefore$ $5 c_1-4 c_2+c_3=27$ ...........(iii) <br/><br/>From (i), (ii) & (iii) <br/><br/>$\vec{c}=3 \hat{i}-2 \hat{j}+4 \hat{k}$ <br/><br/>$$ \begin{aligned} & |\vec{a} \times \vec{c}|^2=\left|\begin{array}{ccc} \hat{i} & \hat{j} & \hat{k} \\ 1 & 2 & 3 \\ 3 & -2 & +4 \end{array}\right|^2 \\\\ & =|14 \hat{i}+5 \hat{j}-8 \hat{k}|^2 \\\\ & =14^2+5^2+8^2=285 \end{aligned} $$

About this question

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

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