cos A +cos B - cos C =
4 cos A/2 *cos B/2* sin C/2 -1
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Answer:A+B+C=25 (Given)
Also,
A
+
B
+
C
=
π
(Sum of angles of triangle)
L.H.S
⇒
cos
(
s
−
A
)
+
cos
(
s
−
B
)
+
cos
(
s
−
C
)
⇒
cos
(
90
°
−
A
)
+
cos
(
90
°
−
B
)
+
cos
(
90
°
−
C
)
⇒
sin
A
+
sin
B
+
sin
C
⇒
2
sin
(
A
+
B
2
)
cos
(
A
−
B
2
)
+
sin
C
[
∵
sin
A
+
sin
B
=
2
sin
(
A
+
B
2
)
cos
(
A
−
B
2
)
]
⇒
2
cos
C
2
cos
(
A
−
B
2
)
+
2
sin
C
2
cos
C
2
⇒
2
cos
C
2
[
cos
(
A
−
B
2
)
+
cos
(
A
+
B
2
)
]
[
∵
sin
C
2
=
cos
(
A
+
B
2
)
]
⇒
2
cos
C
2
×
2
cos
(
A
2
)
cos
(
B
2
)
⇒
4
cos
(
A
2
)
cos
(
B
2
)
cos
(
C
2
)
L.H.S=R.H.S
Hence, Proved.
Step-by-step explanation:
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