Question Number 76777 by aliesam last updated on 30/Dec/19
$${prove}\:{that} \\ $$$$ \\ $$$$\sqrt[{\mathrm{3}}]{{cos}\left(\mathrm{40}°\right)}\:+\:\sqrt[{\mathrm{3}}]{{cos}\left(\mathrm{80}°\right)}\:−\:\sqrt[{\mathrm{3}}]{{cos}\left(\mathrm{20}°\right)}\:=\sqrt[{\mathrm{3}}]{\frac{\mathrm{3}}{\mathrm{2}}\left(\sqrt[{\mathrm{3}}]{\mathrm{9}}−\mathrm{2}\right)} \\ $$