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Mathematics
Mathematics
Problem
The value of
∑
r
=
0
10
cos
3
(
π
r
3
)
\sum_{r=0}^{10} \cos^3\left(\frac{\pi r}{3}\right)
∑
r
=
0
10
cos
3
(
3
π
r
)
is equal to: (a)
−
7
8
-\frac{7}{8}
−
8
7
(b)
−
9
8
-\frac{9}{8}
−
8
9
(c)
−
3
8
-\frac{3}{8}
−
8
3
(d)
−
1
8
-\frac{1}{8}
−
8
1
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