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Byju's Answer
Standard XII
Mathematics
Applications of Dot Product
lf in triangl...
Question
lf in triangle
A
B
C
,
2
cos
A
a
+
cos
B
b
+
2
cos
C
c
=
a
b
c
+
b
c
a
, then the value of the angle
A
, in degrees is
A
30
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B
60
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C
90
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D
120
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Solution
The correct option is
B
90
cos
A
=
b
2
+
c
2
−
a
2
2
b
c
,
cos
B
=
a
2
+
c
2
−
b
2
2
a
c
,
cos
C
=
a
2
+
b
2
−
c
2
2
a
b
2
(
b
2
+
c
2
−
a
2
)
2
a
b
c
+
a
2
+
c
2
−
b
2
2
a
b
c
+
2
(
a
2
+
b
2
−
c
2
)
2
a
b
c
=
a
2
+
b
2
a
b
c
2
b
2
+
2
c
2
−
2
a
2
+
a
2
+
c
2
−
b
2
+
2
a
2
+
2
b
2
−
2
c
2
2
=
a
2
+
b
2
3
b
2
+
a
2
+
c
2
=
2
a
2
+
2
b
2
a
2
=
b
2
+
c
2
∴
∠
A
=
π
2
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0
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A
a
+
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o
s
B
b
+
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c
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