Question Number 185861 by cortano1 last updated on 29/Jan/23
$$\:\:\:\:\mathrm{cos}\:\left(\frac{\mathrm{2}\pi}{\mathrm{13}}\right)+\mathrm{cos}\:\left(\frac{\mathrm{6}\pi}{\mathrm{13}}\right)+\mathrm{cos}\:\left(\frac{\mathrm{8}\pi}{\mathrm{13}}\right)=? \\ $$
Commented by Frix last updated on 29/Jan/23
$$\frac{−\mathrm{1}+\sqrt{\mathrm{13}}}{\mathrm{4}} \\ $$
Commented by cortano1 last updated on 29/Jan/23
$${how} \\ $$