Let the vectors 
, ,
a b c
rr
r  be given as 
1
2
3
1
2
3
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
,
,
a i
a j
a k b i
b j
b k
+
+
+
+
1
2
3 ˆ
ˆ
ˆ
c i
c j
c k
+
+ Then show that 
(
)
a
b
c
a b
a
c
×
+
= ×
+
×
r
r
r
r
r
r
r 8 If either 
0  or  
0,
a
b
=
r=
r
r
r
 then 
0
a
×b
=
r
r
r