If 
0  and  
0
a a
a b
⋅
=
⋅
=
r
r r
r
, then what can be concluded about the vector b
r 13 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