A line has direction ratios $1, -2, 2$. What angle does it make with the $x$-axis?

Explanation

1. 01

   $\sqrt{1 + 4 + 4} = 3$, so $l = \frac{1}{3}$.

2. 02

   The angle with the $x$-axis is $\cos^{-1}\frac{1}{3}$.