Question Number 135603 by liberty last updated on 14/Mar/21
$$\mathrm{cos}\:\left(\frac{\mathrm{2}\pi}{\mathrm{17}}\right)+\mathrm{cos}\:\left(\frac{\mathrm{4}\pi}{\mathrm{17}}\right)+\mathrm{cos}\:\left(\frac{\mathrm{8}\pi}{\mathrm{17}}\right)+\mathrm{cos}\:\left(\frac{\mathrm{16}\pi}{\mathrm{17}}\right)=? \\ $$