Question
Find the vector equation of the line passing through the point having position vector $2 \hat{i}+\hat{j}-3 \hat{k}$ and perpendicular to vectors $\hat{i}+\hat{j}+\hat{k}$. and $\hat{i}+2 \hat{j}-\hat{k}$

Answer

Let $a=2 \hat{i}+\hat{j}-3 \hat{k}, \bar{b}=\hat{i}+\hat{j}+\hat{k}$ and $\mathrm{c}=\hat{i}+2 \hat{j}-\hat{k}$
We know that $\bar{b} \times \bar{c}$ is perpendicular to both $\bar{b}$ and $\bar{c}$.
$\therefore \bar{b} \times \bar{c}$ is parallel to the required line.
$
\bar{b} \times \bar{c}=\left|\begin{array}{ccc}
\hat{i} & \hat{j} & \hat{k} \\
1 & 1 & 1 \\
1 & 2 & -1
\end{array}\right|=-3 \hat{i}+2 \hat{j}+\hat{k}
$
Thus required line passes through $a=2 \hat{i}+\hat{j}-3 \hat{k}$ and parallel to $-3 \hat{i}+2 \hat{j}+\hat{k}$.
$\therefore$ Its equation is $\bar{r}=(2 \hat{i}+\hat{j}-3 \hat{k})+\lambda(-3 \hat{i}+2 \hat{j}+\hat{k})$

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