The value of \( \cos \left(\frac{2 \pi}{7}\right)+\cos \left(\frac{4 \pi}{7}\right)+\cos \left(\...
The value of \( \cos \left(\frac{2 \pi}{7}\right)+\cos \left(\frac{4 \pi}{7}\right)+\cos \left(\frac{6 \pi}{7}\right) \) is equal to:
(a) -1
(b) \( -\frac{1}{2} \)
(c) \( -\frac{1}{3} \)
(d) \( -\frac{1}{4} \)
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