# 13. Find the equation of the plane passing through the point (– 1, 3, 2) and perpendicular to each of the planes x + 2y + 3z = 5 and 3x + 3y + z = 0.

Given

two planes x + 2y + 3z = 5 and 3x + 3y + z = 0.

the normal vectors  of these plane are

$n_1=\hat i+2\hat j+ 3\hat k$

$n_2=3\hat i+3\hat j+ \hat k$

Since the normal vector of the required plane is perpendicular to the normal vector of given planes, the required plane's normal vector will be :

$\vec n = \vec n_1\times\vec n_2$

$\vec n = (\hat i+2\hat j +3\hat k )\times (3\hat i + 3\hat j +\hat k)$

$\vec n =\begin{vmatrix} \hat i &\hat j &\hat k \\ 1 &2 &3 \\ 3& 3& 1 \end{vmatrix}=\hat i(2-9)-\hat j(1-9)+\hat k (3-6)$

$\vec n =-7\hat i+8\hat j-3\hat k$

Now, as we know

the equation of a plane in vector form is :

$\vec r\cdot\vec n=d$

$\vec r\cdot(-7\hat i+8\hat j-3\hat k)=d$

Now Since this plane passes through the point (-1,3,2)

$(-\hat i+3\hat j+2\hat k)\cdot(-7\hat i+8\hat j-3\hat k)=d$

$7+24-6=d$

$d=25$

Hence the equation of the plane is

$\vec r\cdot(-7\hat i+8\hat j-3\hat k)=25$

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