18
© NCERT
not to be republished
 MATHEMATICS
442
defined as
a b
⋅
r
r
 = |
| |
| cos ,
a
b
θ
r
r
where, θ is the angle between 
and
, 0
a
b
   
r
r
 (Fig 10 19) If either 
0 or
0,
a
b
=
=
r
r
r
r
 then θ is not defined, and in this case,
we define 
0
a b
 
r 
r
Observations
1 a b
⋅
r
r
 is a real number