Question

If the vectors $2\mathbf{i} - 3\mathbf{j} + 4\mathbf{k},\mathbf{i} + 2\mathbf{j} - \mathbf{k}$ and $x\mathbf{i} - \mathbf{j} + 2\mathbf{k}$ are coplanar, then $x =$

• A. $\frac{8}{5}$
• B. $\frac{5}{8}$
• C. 0
• D. 1

Answer 1

Answer: (A)

$\frac{8}{5}$

Answer 2

Given vectors are coplanar,

$\therefore\left| \begin{matrix} 2 & - 3 & 4 \\ 1 & 2 & - 1 \\ x & - 1 & 2 \end{matrix} \right| = 0 \Rightarrow x = \frac{8}{5}$.