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 As l1, l2 are coplanar, if the foot of the perpendicular
from T on the line l1 is P, then the distance between the
lines l1 and l2 =  |TP |