An aeroplane is flying along the line `vec r = lambda (hat i - hat j + hat k)`, where `lambda` ' is a scalar and another aeroplane is flying along the line `vec r = hat i - hat j + mu (-2 hat j + hat k)`, where `mu` is a scalar. At what points on the lines should they reach, so that the distance between them is the shortest? Find the shortest possible distance between them.