What is the value of ∑k=1n−1cot2(kπn)\sum_{k=1}^{n-1} \cot^2\left(\frac{k\pi}{n}\right)∑k=1n−1cot2(nkπ)?
(n−1)(n−2)3\frac{(n-1)(n-2)}{3}3(n−1)(n−2)
n(n−1)6\frac{n(n-1)}{6}6n(n−1)
(n−1)(2n−1)6\frac{(n-1)(2n-1)}{6}6(n−1)(2n−1)
n26\frac{n^2}{6}6n2