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Test Index
CBSE Class XII Maths 2015 Question Papers with Solutions
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Question : 2
Total: 26
Validate
Solution:
Let
→
a
=
^
i
−
^
j
;
→
b
=
^
j
−
^
k
→
a
.
→
b
=
(
^
i
−
^
j
)
.
(
^
j
.
^
k
)
= 1 × 0 + ( 1) × 1 + 0 × ( 1) =  1

→
a

=
√
1
2
+
(
−
1
)
2
+
0
2
=
√
2

→
b

=
√
0
2
+
1
2
+
(
−
1
)
2
=
√
2
We know that
→
a
.
→
b
=

→
a


→
b

cos θ
Thus, cos θ =
→
a
.
→
b

→
a


→
b

=
−
1
√
2
×
√
2
= 
1
2
⇒ cos θ = cos 120°
⇒ θ = 120°
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