The acute angle between the two lines whose dc’s are given by  $l+m-n=0$ and  $l^2+m^2-n^2=0$ is

$\begin{array}{rl} & l+m-n=0\\ & \Rightarrow n=l+m\to \left(1\right)\\ & l^2+m^2-(l+m{)}^2=0\end{array}$
$\Rightarrow 2lm=0$
$l=0\left(or\right)m=0$
$\text{If }l=0 \text{then }n=m$
$l:m:n=0:m:m$
$\begin{array}{rl} & =0:1:1\\ & \text{If}m=0\text{then}n=l\\ & l:m:n=l:0:l\\ & =1:0:1\end{array}$