9
© NCERT
not to be republished
VECTOR ALGEBRA
431
Proof Consider the parallelogram ABCD
(Fig 10 10) Let AB
and BC
,
a
b
 
 
uuur
uuur
r
r
 then using
the triangle law, from triangle ABC, we have
AC
uuur  =
a+
rb
r
Now, since the opposite sides of a
parallelogram are equal and parallel, from
Fig 10 10, we have, AD = BC = b
uuur
uuur
r
 and
DC = AB = a
uuur
uuur
r