First slide

Direction ratios and direction cosines

Question

The dc’s $(l, m, n)$ of two lines are connected by the relations $l + m + n = 0,$ $l m = 0$ then the angle between the lines is

Moderate

A

$\frac{π}{3}$
B

$\frac{π}{4}$
C

$\frac{π}{2}$
D

$\frac{π}{6}$

Solution

l=0,subtute l+m+n=0
m=-n
m=0,subtute l+m+n=0
l=-nD.r’s of two lines are $(0, - 1, 1) a n d (- 1, 0, 1)$ ,
$\Rightarrow \cos θ = \frac{1}{2} \Rightarrow θ = \frac{π}{3}$

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