Question Number 87250 by john santu last updated on 03/Apr/20
$$\sqrt[{\mathrm{3}\:\:}]{\mathrm{cos}\:\frac{\pi}{\mathrm{7}}}\:+\:\sqrt[{\mathrm{3}\:\:}]{\mathrm{cos}\:\frac{\mathrm{3}\pi}{\mathrm{7}}}\:+\:\sqrt[{\mathrm{3}\:\:}]{\mathrm{cos}\:\frac{\mathrm{5}\pi}{\mathrm{7}}}\:=? \\ $$$$ \\ $$