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The angle between the lines whose direction cosines satisfy the equations $l+m+n=0,l^2+m^2-n^2=0$ is :

$\begin {array} {1 1} (1)\;\frac{\pi}{6} & \quad (2)\;\frac{\pi}{4} \\ (3)\;\frac{\pi}{3} & \quad (4)\;\frac{\pi}{2} \end {array}$
Can you answer this question?

$(3)\;\frac{\pi}{3}$
answered Nov 7, 2013 by