9), then their
sum 
a+
rb
r
 is represented in magnitude and
direction by the diagonal of the parallelogram
through their common point This is known as
the parallelogram law of vector addition �Note    From Fig 10 9, using the triangle law, one may note that
OA
AC
+
uuur
uuur  = OC
uuur
or
OA
OB
+
uuur
uuur  = OC
uuur                                 (since AC
uuur=OB
uuur
)
which is parallelogram law