The value of \( \sum_{k=1}^{6}\left(\sin \left(\frac{2 \pi k}{7}\right)-i \cos \left(\frac{2 \pi...
The value of \( \sum_{k=1}^{6}\left(\sin \left(\frac{2 \pi k}{7}\right)-i \cos \left(\frac{2 \pi k}{7}\right)\right) \), where \( i=\sqrt{-1} \), is
(a) \( -1 \)
(b) 0
(c) \( -i \)
(d) \( i \)
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