Solution
\[
\begin{aligned}
& 4 \ell+\mathrm{m}-\mathrm{n}=0 \Rightarrow \mathrm{n}=4 \ell+\mathrm{m} \\
& \mathrm{n}(2 \mathrm{~m}+10 \ell)+3 \ell \mathrm{~m}=0 \\
& \Rightarrow(4 \ell+\mathrm{m})(2 \mathrm{~m}+10 \ell)+3 \ell \mathrm{~m}=0 \\
& \Rightarrow 8 \ell \mathrm{~m}+40 \ell^{2}+2 \mathrm{~m}^{2}+10 \ell \mathrm{~m}+3 \ell \mathrm{~m}=0 \\
& \Rightarrow 40 \ell^{2}+16 \ell \mathrm{~m}+5 \ell \mathrm{~m}+2 \mathrm{~m}^{2}=0 \\
& \Rightarrow 8 \ell(5 \ell+2 \mathrm{~m})+\mathrm{m}(5 \ell+2 \mathrm{~m})=0 \\
& \Rightarrow(5 \ell+2 \mathrm{~m})(8 \ell+\mathrm{m})=0
\end{aligned}
\]
\[
\begin{array}{l|l}
5 \mathrm{l}+2 \mathrm{~m}=0 & 8 \ell+\mathrm{m}=0 \\
\Rightarrow \mathrm{~m}=-\frac{5 \ell}{2} & \Rightarrow \mathrm{~m}=-8 \ell \\
\mathrm{n}=4 \ell+\mathrm{m} & \mathrm{n}=4 \ell+\mathrm{m}=4 \ell-8 \ell \\
\mathrm{n}=4 \ell-\frac{5 \ell}{2}=\frac{3 \ell}{2} & \Rightarrow \mathrm{n}=-4 \ell \\
\ell, \mathrm{~m}, \mathrm{~m} \rightarrow \ell,-\frac{5 \ell}{2}, \frac{3 \ell}{2} & \ell, \mathrm{~m}, \mathrm{n} \rightarrow \ell,-8 \ell,-4 \ell
\end{array}
\]
or line $\mathrm{L}_{1} \mathrm{drs} 2,-5,3 \mid$ line $\mathrm{L}_{2} \mathrm{drs}-1,8,4$
\[
\cos \theta=\left|\frac{2(-1)-5(8)+3(4)}{\sqrt{4+25+9} \sqrt{1+64+16}}\right|=\frac{30}{\sqrt{38} \times 9}=\frac{10}{3 \sqrt{38}} \operatorname{Answer}(4)
\]
.
