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 But
ST
uur
=
2
1
a
a−
r
r
Fig 11