2 Distance between parallel lines
If two lines l1 and  l2 are parallel, then they are coplanar Let the lines be given by
rr  =
1a
b
+ λ
r
r (1)
and
rr  =
2a
b
+ µ
r
r
… (2)
where, 
1ar is the position vector of a point S on l1 and
2
ar
 is the position vector of a point T on l2 Fig 11 9