\[\begin{array}{l} {\text{Докажите тождество:}} \hfill \\ {\text{co}}{{\text{s}}^3}\frac{\pi }{7} - {\text{co}}{{\text{s}}^3}\frac{{2\pi }}{7} + {\text{co}}{{\text{s}}^3}\frac{{3\pi }}{7} = \frac{1}{2}. \hfill \\ \end{array}\]