The equations of the two lines are
                                      ...(1)
and                         ...(2)
Comparing these equations with
              
Let S be point on line (1) with position vector  and T be point on line (2) with position vector  so that

                        
Now,          
                                


Let  be the S.D. vector between given lines. Therefore, it is parallel to 
If  is a unit vector along , then
                 
Now, S.D.  =