A= biểu thức
=>$2sin \frac{\pi}{11}A=sin \frac{2\pi}{11}+sin \frac{4\pi}{11}-sin \frac{2\pi}{11}+sin \frac{6\pi}{11}-sin \frac{4\pi}{11}+sin \frac{8\pi}{11}-sin \frac{6\pi}{11}+sin \frac{10\pi}{11}-sin \frac{8\pi}{11}=sin \frac{10\pi}{11}=sin (\pi-\frac{10\pi}{11})=sin\frac{\pi}{11}$
=>A=1/2