If 
, ,
a b c
rr
r  are unit vectors such that 
0
a
b
c
+
+
=
r
r
r
r
, find the value of
a b
b c
c a
⋅
+
⋅
+
⋅
r
r
r
r
r r 14 If either vector 
0  or 
0,  then  
0
a
b
a b
=
=
⋅
=
r
r
r
r
r
r But the converse need not be
true