# 17.  Find the equation of the plane which contains the line of intersection of the planes $\dpi{100} \overrightarrow{r}.(\widehat{i}+2\widehat{j}+3\widehat{k})-4=0,\overrightarrow{r}.\left ( 2\widehat{i}+\widehat{j}-\widehat{k} \right )+5=0$and which is perpendicular to the plane $\overrightarrow{r}.(5\widehat{i}+3\widehat{j}-6\widehat{k})+8=0$

D Divya Prakash Singh

The equation of the plane passing through the line of intersection of the given plane in  $\overrightarrow{r}.(\widehat{i}+2\widehat{j}+3\widehat{k})-4=0,\overrightarrow{r}.\left ( 2\widehat{i}+\widehat{j}-\widehat{k} \right )+5=0$

$\left [ \vec{r}.(\widehat{i}+2\widehat{j}+3\widehat{k})-4 \right ]+\lambda\left [ \vec{r}.\left ( 2\widehat{i}+\widehat{j}-\widehat{k} \right )+5 \right ] = 0$

$\vec{r}.[(2\lambda+1)\widehat{i}+(\lambda+2)\widehat{j}+(3-\lambda)\widehat{k}]+(5\lambda-4)= 0$       ,,,,,,,,,,,,,(1)

The plane in equation (1) is perpendicular to the plane,

$\vec{r}.\left ( 5\widehat{i}+3\widehat{j}-6\wideahat{k} \right ) +8 = 0$

Therefore $5(2\lambda+1)+3(\lambda+2) -6(3-\lambda) = 0$

$\implies 19\lambda -7 = 0$

$\implies \lambda = \frac{7}{19}$

Substituting $\lambda = \frac{7}{19}$ in equation (1), we obtain

$\implies \vec{r}.\left [ \frac{33}{19}\widehat{i}+\frac{45}{19}\widehat{j}+\frac{50}{19}\widehat{k} \right ] -\frac{41}{19} = 0$

$\implies \vec{r}.(33\widehat{i}+45\widehat{j}+50\widehat{k}) -41 = 0$                     .......................(4)

So, this is the vector equation of the required plane.

The Cartesian equation of this plane can be obtained by substituting $\implies \vec{r}= (x\widehat{i}+y\widehat{j}+z\widehat{k})$ in equation (1).

$(x\widehat{i}+y\widehat{j}+z\widehat{k}).(33\widehat{i}+45\widehat{j}+50\widehat{k}) -41 = 0$

Therefore we get the answer $33x+45y+50z -41 = 0$

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