Then 
0
a
×b
r=
r
r
 if and only if 
a  and  
rb
r
are parallel (or collinear) to each other, i e ,
a
b
×
r
r
 = 0
⇔a b
r
r
r �
Fig 10 23
© NCERT
not to be republished
 MATHEMATICS
450
In particular, 
0
a
×a
=
r
r
r
  and 
(
)
0
a
× −a
=
r
r
r
, since in the first situation, θ = 0
and in the second one, θ = π, making the value of sin θ to be 0