\( \sum_{r=1}^{n-1} \cos ^{2} \frac{r \pi}{n} \) is equal to (a) \(...

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\( \sum_{r=1}^{n-1} \cos ^{2} \frac{r \pi}{n} \) is equal to
(a) \( \frac{n}{2} \)
(b) \( \frac{n-1}{2} \)
(c) \( \frac{n}{2}-1 \)
(d) \( \frac{n+1}{2} \)
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