Given that ‌l+m−n=0 . . . (i) ‌l2+m2−n2=0 ‌‌ Putting in equation (ii), we get ‌ ‌⇒‌‌(m−n)2+m2−n2=0 ‌⇒‌‌m2+n2−2mn+m2−n2=0 ‌⇒‌‌2m2−2mn=0 ‌2m(m−n)=0 ‌⇒‌‌m=0,m=n From equation (i), if m=0, then l=n⇒l:m:n=1:0:1 If m=n, then l=0⇒l:m:n=0:1:1 If θ is the acute angle, then cos‌θ‌=|‌