Substituting n=−l−m in 2l2+2m2−n2=0, we get ‌2l2+2m2−(−l−m)2=0 ‌⇒l2+m2−2lm=0 ‌⇒(l−m)2=0 ‌⇒l=m If l=m, then n=−2m ⇒‌
l
1
=‌
m
1
=‌
n
−2
The direction ratios of both the lines are same. ‌∴‌‌cos‌θ=±1 ‌⇒θ=0∘‌ or ‌180∘