In terms of vector product, the angle between two vectors   and  
a
rb
r
  may be
given as
sin θ = |
|
|
||
|
a
b
a b
×
r
r
r
r
6 It is always true that the vector product is not commutative, as a
×b
r
r
 = 
b
a
−
r×
r Indeed, 
ˆ
|
||
| sin
a
b
a b
n
×
=
θ
r
r
r
r
, where 
ˆ
,
a b and  
n
rr
 form a right handed system,
i e