Cos7x=(√1- x^{2} )^2+ x^{2}

[latex]cos7x = (\sqrt{1 - x^2} )^2 + x^2 \\ cos7x = 1 - x^2 + x^2 \\ cos7x = 1 \\ 7x = 2 \pi n, \ n \in Z \\ \\ x = \dfrac{2 \pi n}{7}, \ n \in Z, \ x \in [-1; 1] - eto \ ODZ \\ \\ x = \pm \dfrac{2 \pi }{7}, \ 0.[/latex]