Answer:
Option A
Explanation:
We konow that angle berween two lines is
\cos \theta=\frac{a_{1}a_{2}+b_{1}b_{2}+c_{1}c_{2}}{\sqrt{a_1^2+b_1^2+c_1^2}\sqrt{a_2^2+b_2^2+c_2^2}}
l+m+n=0
\Rightarrow l=-(m+n)
\Rightarrow (m+n)^{2}=l^{2}
\Rightarrow m^{2}+n^{2}+2mn=m^{2}+n^{2}
[ \because l^{2}=m^{2}+n^{2},given]
\Rightarrow 2mn=0
when m=0 \Rightarrow l=-n
Hence, (l,m,n) is (1,0,-1)
when n=0, then l=-m
Hence , (l,m,n) is (1,0,-1)
\therefore \cos\theta=\frac{1+0+0}{\sqrt{2}\times\sqrt{2}}
= \frac{1}{2}
\Rightarrow θ=\frac{\pi}{3}