Question

The angle between the lines whose direction cosines satisfy the equations $l+m+n=0,l^2+m^2-n^2=0$ is given by
[MP PET 1993; RPET 2001]

• A.
  
• B.
  
• C.
  
• D.
  

Answer 1

The correct option is D

$l+m+n=0,l^2+m^2-n^2=0and{l}^2+m^2+n^2=1$
Solving above equations, we get
$m=\pm \frac{1}{\sqrt{2}},n=\pm \frac{1}{\sqrt{2}}andl=0.\therefore \theta =\frac{\pi }{3}or\frac{\pi }{2}$