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Problem
Problem
Show that in any
Δ
A
B
C
\Delta ABC
Δ
A
B
C
,
a
3
cos
3
B
+
3
a
2
b
cos
(
2
B
−
A
)
+
3
a
b
2
cos
(
B
−
2
A
)
+
b
3
cos
3
A
=
c
3
a^3 \cos 3B + 3a^2 b \cos(2B - A) + 3ab^2 \cos(B - 2A) + b^3 \cos 3A = c^3
a
3
cos
3
B
+
3
a
2
b
cos
(
2
B
−
A
)
+
3
a
b
2
cos
(
B
−
2
A
)
+
b
3
cos
3
A
=
c
3
.
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