# The angle between the lines whose direction cosines satisfy the equations $l+m+n=0,l^2+m^2-n^2=0$ is :
$\begin {array} {1 1} (1)\;\frac{\pi}{6} & \quad (2)\;\frac{\pi}{4} \\ (3)\;\frac{\pi}{3} & \quad (4)\;\frac{\pi}{2} \end {array}$
$(3)\;\frac{\pi}{3}$