We have, 1+m−n=0 ‌ and ‌‌‌l2+m2−n2=0 ⇒‌‌l+m=n‌ and ‌l2+m2=n2 ⇒‌‌l2+m2=(l+m)2 ⇒‌‌2lm=0 ⇒l=0‌ or ‌m=0 ‌ If ‌l=0,‌ then ‌m=n ∴‌‌‌
l
0
=‌
m
1
=‌
n
1
If m=0, then l=n ∴‌
l
1
=‌
m
0
=‌
n
1
Thus, the dr's of the lines are proportional to 0,1 1 and 1,0,1. Therefore, angle θ between them is given by cos‌θ=‌