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$\cos\left( \frac{\pi}{11}\right) - \cos\left( \frac{2\pi}{11} \right)+ \cos\left( \frac{3\pi}{11}\right) \\ - \cos\left( \frac{4\pi}{11} \right)+ \cos\left( \frac{5\pi}{11} \right)=\ ?$

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