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 | Let θ be the angle between the vectors ST
uur
and b
r Then
ST
b ×
uur
r
 =
ˆ
(|
b|| ST| sin )
n
θ
uur
r (3)
where nˆ  is the unit vector perpendicular to the plane of the lines l1 and l2