Question

The angle between the lines whose direction cosines are proportional to (1, 2, 1) and (2, -3, 6) is

• A.
  
• B.
  
• C.
  
• D.
  

Answer 1

The correct option is A

$\theta =co{s}^{-1}\left[\frac{\left(1\right)\left(2\right)+\left(2\right)(-3)+\left(1\right)\left(6\right)}{\sqrt{1^2+2^2+1^2}\sqrt{2^2+(-3{)}^2+6^2}}\right]co{s}^{-1}\left[\frac{2-6+6}{\sqrt{6}\sqrt{49}}\right]=co{s}^{-1}\left[\frac{2}{7\sqrt{6}}\right].$