Let the lines
$\frac{x-1}{\lambda}=\frac{y-2}{1}=\frac{z-3}{2}$ and
$\frac{x+26}{-2}=\frac{y+18}{3}=\frac{z+28}{\lambda}$ be coplanar
and $\mathrm{P}$ be the plane containing these two lines.
Then which of the following points does NOT lie on P?
Solution
<p>${L_1}:{{x - 1} \over \lambda } = {{y - 2} \over 1} = {{z - 3} \over 2}$,</p>
<p>through a point ${\overrightarrow a _1} \equiv (1,2,3)$</p>
<p>parallel to ${\overrightarrow b _1} \equiv (\lambda ,1,2)$</p>
<p>${L_2}:{{x + 26} \over { - 2}} = {{y + 18} \over 3} = {{z + 28} \over \lambda }$</p>
<p>through a point ${\overrightarrow a _2} = ( - 26, - 18, - 28)$</p>
<p>parallel to ${\overrightarrow b _2} = ( - 2,3,1)$</p>
<p>If lines are coplanar then, $$({\overrightarrow a _2} - {\overrightarrow a _1})\,.\,{\overrightarrow b _1} \times {\overrightarrow b _2} = 0$$</p>
<p>$$ \Rightarrow \left| {\matrix{
{27} & {20} & {31} \cr
\lambda & 1 & 2 \cr
{ - 2} & 3 & \lambda \cr
} } \right| = 0 \Rightarrow \lambda = 3$$</p>
<p>Vector normal to the required plane $\overrightarrow n = {\overrightarrow b _1} \times {\overrightarrow b _2}$</p>
<p>$$ \Rightarrow \overrightarrow n = \left| {\matrix{
{\widehat i} & {\widehat j} & {\widehat k} \cr
3 & 1 & 2 \cr
{ - 2} & 3 & 3 \cr
} } \right| = - 3\widehat i - 13\widehat j + 11\widehat k$$</p>
<p>Equation of plane</p>
<p>$\equiv ((x - 1),(y - 2),(z - 3))\,.\,( - 3, - 13,11) = 0$</p>
<p>$\Rightarrow 3x + 13y - 11z + 4 = 0$</p>
<p>Checking the option gives (0, 4, 5) does not lie on the plane.</p>
About this question
Subject: Mathematics · Chapter: Three Dimensional Geometry · Topic: Direction Cosines and Ratios
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