It is given that, l+m+n=0 .....(I) l+m=−n −(l+m)=n And, l2+m2−n2=0 ........(2) Consider equation (1) and (2), l2+m2−l2−m2−2ml=0 2ml=0 This implies either m or l is zero. Let assume m=0 then, l=−n And, The direction ratios are (l,m,n)=(1,0,−1) Let assume l=0 then, m=−n And, The direction ratios are (l,m,n)=(0,1,−1) Here, b1.b2=(1,0,−1)(0,1,−1) =0+0+1 This implies, (b1)=√02+12+12 =√2 And, (b2)=√02+12+12 =√2 So, the angle is given by, cos‌θ=