CMR : $cos\frac{2\pi}{2n+1} +cos\frac{4\pi }{2n+1}+...+cos\frac{2n\pi }{2n+1}=\frac{-1}{2}$
Ta có:
$2\sin \frac{\pi}{2n+1}.S=\sin\frac{3\pi}{2n+1}-\sin\frac{\pi}{2n+1}+\sin\frac{5\pi}{2n+1}-\sin\frac{3\pi}{2n+1}+...+\sin\pi-\sin\frac{(2n-1)\pi}{2n+1}$
$=-\sin\frac{\pi}{2n+1}$
$\\ \Leftrightarrow S=\frac{-1}{2}$
- LzuTao yêu thích