IMO Shortlist 1963 problem 5


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Dodao/la: arhiva
April 2, 2012
1963 IMO shortlist
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Prove that \cos{\frac{\pi}{7}}-\cos{\frac{2\pi}{7}}+\cos{\frac{3\pi}{7}}=\frac{1}{2}
Source: Međunarodna matematička olimpijada, shortlist 1963