Question
By computing the shortest distance determine whether the following pairs of lines intersect or not:
$\vec{\text{r}}=\big(\hat{\text{i}}+\hat{\text{j}}-\hat{\text{k}}\big)+\lambda\big(3\hat{\text{i}}-\hat{\text{j}}\big)$ and $\vec{\text{r}}=\big(4\hat{\text{i}}-\hat{\text{k}}\big)+\mu\big(2\hat{\text{i}}+3\hat{\text{k}}\big)$

Answer

Given equations of lines are,
$\vec{\text{r}}=\big(\hat{\text{i}}+\hat{\text{j}}-\hat{\text{k}}\big)+\lambda\big(3\hat{\text{i}}-\hat{\text{j}}\big)$
$\Rightarrow\vec{\text{a}}_1=\big(\hat{\text{i}}+\hat{\text{j}}-\hat{\text{k}}\big),\vec{\text{b}}_1=\big(3\hat{\text{i}}-\hat{\text{j}}\big)$
and, $\vec{\text{r}}=\big(4\hat{\text{i}}-\hat{\text{k}}\big)+\mu\big(2\hat{\text{i}}+3\hat{\text{k}}\big)$
$\Rightarrow\vec{\text{a}}_2=\big(4\hat{\text{i}}-\hat{\text{k}}\big),\vec{\text{b}}_2=\big(2\hat{\text{i}}+3\hat{\text{k}}\big)$
We know that, shortest distance between lines $\vec{\text{r}}=\vec{\text{a}}_1+\lambda\vec{\text{b}}_1$ and $\vec{\text{r}}=\vec{\text{a}}_2+\lambda\vec{\text{b}}_2$ is given by
$\text{S.D.}=\Bigg|\frac{\big(\vec{\text{a}}_2-\vec{\text{a}}_1\big).\big(\vec{\text{b}}_1\times\vec{\text{b}}_2\big)}{\big|\vec{\text{b}}_1\times\vec{\text{b}}_2\big|}\Bigg|\dots(1)$
$\big(\vec{\text{a}}_2-\vec{\text{a}}_1\big)=\big(4\hat{\text{i}}-\hat{\text{k}}\big)-\big(\hat{\text{i}}+\hat{\text{j}}-\hat{\text{k}}\big)$
$=4\hat{\text{i}}-\hat{\text{k}}-\hat{\text{i}}-\hat{\text{j}}+\hat{\text{k}}$
$=3\hat{\text{i}}-\hat{\text{j}}$
$\vec{\text{b}}_1\times\vec{\text{b}}_2=\begin{vmatrix}\hat{\text{i}}&\hat{\text{j}}&\hat{\text{k}}\\3&-1&0\\2&0&3 \end{vmatrix}$
$=\hat{\text{i}}(-3-0)-\hat{\text{j}}(9-0)+\hat{\text{k}}(0+2)$
$=-3\hat{\text{i}}-9\hat{\text{j}}+2\hat{\text{k}}$
$\big|\vec{\text{b}}_1\times\vec{\text{b}}_2\big|=\sqrt{(-3)^2+(-9)^2+(2)^2}$
$\big|\vec{\text{b}}_1\times\vec{\text{b}}_2\big|=\sqrt{9+81+4}$
$=\sqrt{94}$
$\big(\vec{\text{a}}_2-\vec{\text{a}}_1\big).\big(\vec{\text{b}}_1\times\vec{\text{b}}_2\big)=\big(3\hat{\text{i}}-\hat{\text{j}}\big)\big(-3\hat{\text{i}}-9\hat{\text{j}}+2\hat{\text{k}}\big)$
$=(3)(-3)+(-1)(-9)+(0)(2)$
$=-9+9+0$
$=0$
Using $\big(\vec{\text{a}}_2-\vec{\text{a}}_1\big).\big(\vec{\text{b}}_1\times\vec{\text{b}}_2\big)$ and $\big|\vec{\text{b}}_1\times\vec{\text{b}}_2\big|$ in equation (1) to get shortest distance between given lines, so
$\text{S.D.}=\Big|\frac{0}{\sqrt{94}}\Big|$
$\text{S.D.}=0$
Since, shortest distance between the given lines is not zero, so lines are intersecting.

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