IMO 1992 LL SPA68

Show that the numbers tan(r\pi/15), where r is a positive integer

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IMO 1992 LL SPA68

Origin: SPA

Problem

Show that the numbers tan(r\pi/15), where r is a positive integer less than 15 and relatively prime to 15, satisfy x8 −92x6 + 134x4 −28x2 + 1 = 0.