Q. Let $\overrightarrow{A} = \widehat{\mathbf{i}} + \widehat{\mathbf{j}} + \widehat{\mathbf{k}},\ \overrightarrow{\mathbf{B}} = \widehat{\mathbf{i}},\ \overrightarrow{\mathbf{C}} = c_{1}\widehat{\mathbf{i}} + c_{2}\widehat{\mathbf{j}}\mathbf{+}c_{3}\widehat{\mathbf{k}}$. If $c_{2} = - 1$ and $c_{3} = 1$, then to make three vectors coplanar

A) $c_{1} = 0$
B) $c_{1} = 1$
C) $c_{1} = 2$
D) No value of $c_{1}$can be found
Explanation

(d)
Given that, $\overrightarrow{\mathbf{A}} = \widehat{\mathbf{i}}\mathbf{+}\widehat{\mathbf{j}}\mathbf{+}\widehat{\mathbf{k}}\mathbf{,\ }\overrightarrow{\mathbf{B}} = \widehat{\mathbf{i}}\mathbf{,\ }\overrightarrow{\mathbf{C}} = c_{1}\widehat{\mathbf{i}} + c_{2}\widehat{\mathbf{j}} + c_{3}\widehat{\mathbf{k}}$
Since, $\overrightarrow{\mathbf{A}}$, $\overrightarrow{\mathbf{B}}$, $\overrightarrow{\mathbf{C}}$ are coplanar.
$\therefore\lbrack\overrightarrow{\mathbf{A}}$ $\overrightarrow{\mathbf{B}}$ $\overrightarrow{\mathbf{C}}$]=0
Now, $\overrightarrow{\mathbf{B}}$ $\mathbf{\times}\overrightarrow{\mathbf{C}} = \left| \begin{matrix}
\widehat{\mathbf{i}} & \widehat{\mathbf{j}} & \widehat{\mathbf{k}} \\
1 & 0 & 0 \\
c_{1} & c_{2} & c_{3}
\end{matrix} \right| = - c_{3}\widehat{\mathbf{j}} + c_{2}\widehat{\mathbf{k}}$
$\therefore\ \overrightarrow{\mathbf{A}}\mathbf{\bullet (}\overrightarrow{\mathbf{B}}$ $\mathbf{\times}\overrightarrow{\mathbf{C}}\mathbf{) =}\left( \widehat{\mathbf{i}}\mathbf{+}\widehat{\mathbf{j}}\mathbf{+}\widehat{\mathbf{k}} \right) \bullet \left( - c_{3}\widehat{\mathbf{j}} + c_{2}\widehat{\mathbf{k}} \right) = 0$
$\Rightarrow \$No value of $c_{1}$ can be found.

Dr. Abubacker
Reviewed by Dr. Abubacker

Physics Educator & NEET/JEE Specialist

Ph.D. in Physics with 19+ years of teaching experience.

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