Question Number 85582 by jagoll last updated on 23/Mar/20
$$\mathrm{cos}\:\left(\frac{\pi}{\mathrm{9}}\right)+\mathrm{cos}\:\left(\frac{\mathrm{2}\pi}{\mathrm{9}}\right)+\mathrm{cos}\:\left(\frac{\mathrm{4}\pi}{\mathrm{9}}\right)= \\ $$
Commented by som(math1967) last updated on 23/Mar/20
$$\mathrm{2}{cos}\left(\frac{\pi}{\mathrm{9}}\right) \\ $$