# 15.  Find the equation of the plane passing through the line of intersection of the planes $\overrightarrow{r}.\left ( \widehat{i}+\widehat{j}+\widehat{k} \right )=1$ and $\overrightarrow{r}.\left (2 \widehat{i}+3\widehat{j}-\widehat{k} \right )+4=0$   and parallel to x-axis.

So, the given planes are:

$\overrightarrow{r}.\left ( \widehat{i}+\widehat{j}+\widehat{k} \right )=1$  and  $\overrightarrow{r}.\left (2 \widehat{i}+3\widehat{j}-\widehat{k} \right )+4=0$

The equation of any plane passing through the line of intersection of these planes is

$[\overrightarrow{r}.\left ( \widehat{i}+\widehat{j}+\widehat{k} \right )-1] + \lambda \left [ \overrightarrow{r}.\left (2 \widehat{i}+3\widehat{j}-\widehat{k} \right )+4\right ] = 0$

$\vec{r}.[(2\lambda+1)\widehat{i}+(3\lambda+1)\widehat{j}+(1-\lambda)\widehat{k}]+(4\lambda+1) = 0$           ..............(1)

Its direction ratios are $(2\lambda+1) , (3\lambda+1),$  and $(1-\lambda)$  = 0

The required plane is parallel to the x-axis.

Therefore, its normal is perpendicular to the x-axis.

The direction ratios of the x-axis are 1,0, and 0.

$\therefore \1.(2\lambda+1) + 0(\3\lambda+1)+0(1-\lambda) = 0$

$\implies 2\lambda+1 = 0$

$\implies \lambda = -\frac{1}{2}$

Substituting $\lambda = -\frac{1}{2}$ in equation (1), we obtain

$\implies \vec{r}.\left [ -\frac{1}{2}\widehat{j}+\frac{3}{2}\widehat{k} \right ]+(-3)= 0$

$\implies \vec{r}(\widehat{j}-3\widehat{k})+6= 0$

So, the Cartesian equation is $y -3z+6 = 0$.

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