Let   and 
a
rb
r
 be two unit vectors and θ is the angle between them Then a
+b
r
r
is a unit vector if
(A)
θ =π4
(B)
θ =π3
(C)
θ =π2
(D)
π32
θ =
18 The value of 
ˆ
ˆ
ˆ
ˆ ˆ
ˆ
ˆ
ˆ
ˆ (
)
(
)
(
)
i
j
k
j
i
k
k
i
j
 
 
 
 
 
 
 
 is
(A) 0
(B) –1
(C) 1
(D) 3
19