hello,I have to find Direction Cosines(Dcs) of third line(l,m,n), given Dcs of two lines (l1,m1,n1) and (l2,m2,n2). If I had the case as : l*l1+m*m1+n*n1=0; l*l2+m*m2+n*n2=0 (Here both eqns , RHS=0) By cross multiplying I could have found (l,m,n) as as : l/(m1*n2-n1*m2) = m/(n1*l2-l1*n2) = n/(l1*m2-m1*l2); I will get Direction ratios , then I will normalize to get Dcs. Similarly I have to find (l,m,n) if one RHS is not 0? that is: l*l1+m*m1+n*n1=0; l*l2+m*m2+n*n2=cos(theta) . This theta I have. So how should I proceed to find (l,m,n). Thanks.