The equations of 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. = Projection of