8 (iii)), i e ,
BC′
uuuur
 =
BC
−
uuur
Then, on applying triangle law from the Fig 10 8 (iii), we have
AC
AB
BC
′
′
=
+
uuuur
uuur
uuuur  = AB
( BC)
+ −
uuur
uuur
a
b
=
−
r
r
The vector AC′
uuuur  is said to represent the difference of 
aand
rb
r