# 14. If the points (1, 1, p) and (– 3, 0, 1) be equidistant from the plane $\overrightarrow{r}.(3\widehat{i}+4\widehat{j}-12\widehat{k})+13 =0$ then find the value of p.

D Divya Prakash Singh

Given that the points $A(1,1,p)$ and $B(-3,0,1)$ are equidistant from the plane

$\overrightarrow{r}.(3\widehat{i}+4\widehat{j}-12\widehat{k})+13 =0$

So we can write the position vector through the point $(1,1,p)$ is $\vec{a_{1}} = \widehat{i}+\widehat{j}+p\widehat{k}$

Similarly, the position vector through the point $B(-3,0,1)$ is

$\vec{a_{2}} = -4\widehat{i}+\widehat{k}$

The equation of the given plane is $\overrightarrow{r}.(3\widehat{i}+4\widehat{j}-12\widehat{k})+13 =0$

and We know that the perpendicular distance between a point whose position vector is $\vec{a}$  and the plane, $\vec{n} = 3\widehat{i}+4\widehat{j}-12\widehat{k}$  and $d =-13$

Therefore, the distance between the point $A(1,1,p)$ and the given plane is

$D_{1} = \frac{\left | (\widehat{i}+\widehat{j}+p\widehat{k}).(3\widehat{i}+4\widehat{j}-12\widehat{k})+13 \right |}{3\widehat{i}+4\widehat{j}-12\widehat{k}}$

$D_{1} = \frac{\left | 3+4-12p+13 \right |}{\sqrt{3^2+4^2+(-12)^2}}$

$D_{1} = \frac{\left | 20-12p \right |}{13}$                                 .........................(1)

Similarly, the distance between the point $B(-1,0,1)$, and the given plane is

$D_{2} = \frac{\left | (-3\widehat{i}+\widehat{k}).(3\widehat{i}+4\widehat{j}-12\widehat{k})+13 \right |}{3\widehat{i}+4\widehat{j}-12\widehat{k}}$

$D_{2} = \frac{\left |-9-12+13 \right |}{\sqrt{3^2+4^2+(-12)^2}}$

$D_{2} = \frac{8}{13}$                                                             .........................(2)

And it is given that the distance between the required plane and the points, $A(1,1,p)$  and  $B(-3,0,1)$   is equal.

$\therefore D_{1} =D_{2}$

$\implies \frac{\left | 20-12p \right |}{13} =\frac{8}{13}$

therefore we have,

$\implies 12p =12$

or $p =1$  or  $p = \frac{7}{3}$

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