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# Find the Angle Between the Lines Whose Direction Cosines Are Given by the Equations 2l + 2m − N = 0, Mn + Ln + Lm = 0 - CBSE (Arts) Class 12 - Mathematics

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ConceptDirection Cosines and Direction Ratios of a Line

#### Question

Find the angle between the lines whose direction cosines are given by the equations

2l + 2m − n = 0, mn + ln + lm = 0

#### Solution

The given relations are

2l + 2m − n = 0                   .....(1)

mn + ln + lm = 0                 .....(2)

From (1), we have

n = 2l + 2m

Putting this value of n in (2), we get

$m\left( 2l + 2m \right) + l\left( 2l + 2m \right) + lm = 0$

$\Rightarrow 2lm + 2 m^2 + 2 l^2 + 2lm + lm = 0$

$\Rightarrow 2 m^2 + 5lm + 2 l^2 = 0$

$\Rightarrow \left( 2m + l \right)\left( m + 2l \right) = 0$

$\Rightarrow 2m + l = 0 \text{ or m + 2l = 0}$

$\Rightarrow l =\text{ - 2m or l } = - \frac{m}{2}$

$\text{ When l} = - 2m$  we have

$n = 2 \times \left( - 2m \right) + 2m = - 4m + 2m = - 2m$

When  $l = - \frac{m}{2}$ we have

$n = 2 \times \left( - \frac{m}{2} \right) + 2m = - m + 2m = m$

Thus, the direction ratios of two lines are proportional to

$- 2m, m, - 2m$  and $- \frac{m}{2}, m, m$

Or  $- 2, 1, - 2$ and -1,2,2

So, vectors parallel to these lines are $\vec{a} = - 2 \hat{i} + \hat{j} - 2 \hat{k}$ and $\vec{b} = - \hat{i} + 2\hat{j} - 2 \hat{k}$

Let theta be the angle between these lines, then theta is also the anglebetween   vec a and

vec b

$\therefore \cos\theta = \frac{\vec{a} . \vec{b}}{\left| \vec{a} \right|\left| \vec{b} \right|}$

$= \frac{\left( - 2 \hat{i} + \hat{j} - 2 \hat{k} \right) . \left( - \hat{i} + 2 \hat{j} + 2 \hat{k} \right)}{\sqrt{4 + 1 + 4}\sqrt{1 + 4 + 4}}$

$= \frac{- 2 \times \left( - 1 \right) + 1 \times 2 + \left( - 2 \right) \times 2}{3 \times 3}$

$= \frac{2 + 2 - 4}{9}$

$= 0$

$\Rightarrow \theta = \frac{\pi}{2}$

Thus, the angle between the two lines whose direction cosines are given by the given relations is

$\frac{\pi}{2}$

Is there an error in this question or solution?

#### APPEARS IN

RD Sharma Solution for Mathematics for Class 12 by R D Sharma (Set of 2 Volume) (2018-19 Session) (2018 to Current)
Chapter 27: Direction Cosines and Direction Ratios
Ex.27.1 | Q: 16.4 | Page no. 23

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Solution Find the Angle Between the Lines Whose Direction Cosines Are Given by the Equations 2l + 2m − N = 0, Mn + Ln + Lm = 0 Concept: Direction Cosines and Direction Ratios of a Line.
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