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Mathematics
Mathematics
Problem
If
f
n
(
x
)
=
∏
s
=
0
n
−
1
2
cos
(
2
s
x
)
−
1
f_n(x) = \prod_{s=0}^{n-1} 2 \cos(2^s x) - 1
f
n
(
x
)
=
∏
s
=
0
n
−
1
2
cos
(
2
s
x
)
−
1
,
n
≥
1
n \ge 1
n
≥
1
. Then, which of the following is(are) incorrect?
f
5
(
4
π
/
127
)
=
1
f_5(4\pi/127) = 1
f
5
(
4
π
/127
)
=
1
f
5
(
6
π
/
255
)
=
1
f_5(6\pi/255) = 1
f
5
(
6
π
/255
)
=
1
f
5
(
2
π
/
31
)
=
1
f_5(2\pi/31) = 1
f
5
(
2
π
/31
)
=
1
f
5
(
2
π
/
33
)
=
−
1
f_5(2\pi/33) = -1
f
5
(
2
π
/33
)
=
−
1
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