e 1
2
AB (
)
b
b
×
uuur r
r
 = 0 or 
2
1
1
2
(
) (
)
a
a
b
b
−
⋅
×
r
r
r
r
 = 0
Cartesian form
Let (x1, y1, z1) and (x2, y2, z2) be the coordinates of the points A and B respectively © NCERT
not to be republished
 MATHEMATI CS
488
Let a1, b1, c1 and a2, b2, c2 be the direction ratios of 1b
r
and 
2b
r
, respectively