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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