(1)
and
rr  = 
2
2
a
b
+ µ
r
r (2)
The line (1) passes through the point, say A, with position vector 
1
ar and is parallel
to 
1b
r The line (2) passes through the point, say B with position vector 
2
ar and is parallel
to 
2b
r Thus,
AB
uuur
 = 
2
1
a
a−
r
r
The given lines are coplanar if and only if  AB
uuur
 is perpendicular to 1
2
b
b
×
r
r