[tex]Notam~x= \frac{2\pi}{11} ~si~folosind:\\
\displaystyle sinx+sin2x+sin3x+...+sinnx= \frac{sin(n+ \frac{1}{2} )x-sin( \frac{x}{2} )}{2sin \frac{x}{2} } \\
Suma~data= \frac{sin(5+ \frac{1}{2} ) \frac{2\pi}{11}-sin( \frac{ \frac{2\pi}{11}}{2} )}{2sin \frac{ \frac{2\pi}{11}}{2} } =\\
= \frac{sin\pi-sin \frac{\pi}{11} }{2sin \frac{\pi}{11} } = -\frac{1}{2} [/tex]
