The pair of lines whose direction cosines are given by the equations 3l + m + 5n = 0 and  6mn - 2nl + 5lm=0 are

l + m + 5n = 0 --------(i) 6mn - 2nl + 5lm=0--------(ii)Substituting the value of n from (i) in (ii), we get     6l2+9lm−6m2=0 or     6lm2+9lm−6=0∴     l1m1=12 and l2m2=−2From Eq. (i), we get l1n1=−1 and l2n2=−2∴ l11=m12=n1−1=l12+m12+n121+4+1=16 and  l22=m2−1=n2−1=l22+m22+n224+1+1=16