Thus, another unit
vector perpendicular to 
and
a
b
a
b
+
−
r
r
r
r
 will be 1
2
1 ˆ
ˆ
ˆ 6
6
6
i
j
k
−
+
 But that will
be a consequence of (
)
(
)
a
b
a
b
−
×
+
r
r
r
r Example 24 Find the area of a triangle having the points A(1, 1, 1), B(1, 2, 3)
and C(2, 3, 1) as its vertices Solution We have 
ˆ
ˆ
ˆ
ˆ
AB
2
and AC
2
j
k
i
j
=
+
= +
uuur
uuur