Question Number 191050 by Shrinava last updated on 16/Apr/23 | ||
$$\mathrm{Prove}\:\mathrm{that}: \\ $$ $$\mathrm{cos}\:\left(\frac{\mathrm{2}\pi}{\mathrm{13}}\right)\:\centerdot\:\mathrm{cos}\:\left(\frac{\mathrm{5}\pi}{\mathrm{13}}\right)\:\centerdot\:\mathrm{cos}\:\left(\frac{\mathrm{6}\pi}{\mathrm{13}}\right)\:=\:\frac{\sqrt{\mathrm{13}}\:−\:\mathrm{3}}{\mathrm{16}} \\ $$ | ||