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Mathematics
Problem
Prove that
cos
6
∘
cos
42
∘
cos
66
∘
cos
78
∘
=
1
16
\cos 6^\circ \cos 42^\circ \cos 66^\circ \cos 78^\circ = \frac{1}{16}
cos
6
∘
cos
4
2
∘
cos
6
6
∘
cos
7
8
∘
=
16
1
.
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