Let $\mathrm{P}$ be the plane containing the straight line $\frac{x-3}{9}=\frac{y+4}{-1}=\frac{z-7}{-5}$ and perpendicular to the plane containing the straight lines $\frac{x}{2}=\frac{y}{3}=\frac{z}{5}$ and $\frac{x}{3}=\frac{y}{7}=\frac{z}{8}$. If $\mathrm{d}$ is the distance of $\mathrm{P}$ from the point $(2,-5,11)$, then $\mathrm{d}^{2}$ is equal to :
Solution
Let $\langle a, b, c\rangle$ be direction ratios of plane containing
<br/><br/>
$$
\begin{aligned}
&\text { lines } \frac{x}{2}=\frac{y}{3}=\frac{z}{5} \text { and } \frac{x}{3}=\frac{y}{7}=\frac{z}{8} . \\\\
&\therefore \quad 2 a+3 b+5 c=0 \quad \ldots \text { (i) } \\\\
&\text { and } 3 a+7 b+8 c=0 \quad \ldots \text { (ii) } \\\\
&\text { from eq. (i) and (ii) }: \frac{a}{24-35}=\frac{b}{15-16}=\frac{c}{14-9} \\\\
&\therefore \text { D. R's} \text {. of plane are }<11,1,-5>
\end{aligned}
$$
<br/><br/>
Let $D . R$'s of plane $P$ be $< a_{1}, b_{1}, c_{1} >$ then.
<br/><br/>
$11 a_{1}+b_{1}-5 c_{1}=0$
<br/><br/>
and $9 a_{1}-b_{1}-5 c_{1}=0$
<br/><br/>
From eq. (iii) and (iv) :
<br/><br/>
$$
\begin{aligned}
&\frac{a_{1}}{-5-5}=\frac{b_{1}}{-45+55}=\frac{c_{1}}{-11-9} \\\\
&\therefore \text { D.R's } \text { of plane } P \text { are }< 1,-1,2 >
\end{aligned}
$$
<br/><br/>
Equation plane $P$ is : $1(x-3)-1(y+4)+2(z-7)=0$
<br/><br/>
$\Rightarrow x-y+2 z-21=0$
<br/><br/>
Distance from point $(2,-5,11)$ is $d=\frac{|2+5+22-2|}{\sqrt{6}}$<br/><br/> $\therefore d^{2}=\frac{32}{3}$
About this question
Subject: Mathematics · Chapter: Three Dimensional Geometry · Topic: Direction Cosines and Ratios
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