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Byju's Answer
Standard VII
Mathematics
Equal Angles Subtend Equal Sides
In any ABC,...
Question
In any
△
A
B
C
, prove that
a
(
cos
C
−
cos
B
)
=
2
(
b
−
c
)
cos
2
A
2
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Solution
L
H
S
=
a
[
a
2
+
b
2
−
c
2
2
a
b
−
c
2
+
a
2
−
b
2
2
c
a
]
=
a
[
a
2
c
+
b
2
c
−
c
3
−
c
2
b
−
a
2
b
+
b
3
2
a
b
c
]
=
(
b
3
−
c
3
)
+
(
b
2
c
−
b
c
2
)
−
(
a
2
−
a
2
c
)
2
b
c
=
(
b
−
c
)
(
b
2
+
c
2
+
b
c
)
+
b
c
(
b
−
c
)
−
a
2
(
b
−
c
)
2
b
c
=
(
b
−
c
)
b
2
+
c
2
+
b
c
+
b
c
−
a
2
2
b
c
=
(
b
−
c
)
{
b
2
+
c
2
−
a
2
2
b
c
+
2
b
c
2
b
c
}
=
(
b
−
c
)
{
cos
A
+
1
}
=
(
b
−
c
)
2
cos
2
A
2
=
2
(
b
−
c
)
cos
2
A
2
=
R
H
S
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Q.
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