Đáp án`+`Giải thích các bước giải:

Ta có: `cos^2 x = sin^2  (\pi)/7`

`<=> cos^2x - sin^2 (\pi/7) =0`

`<=> (cos x - sin (\pi/7))(cosx + sin(\pi/7))=  0`

`<=>` $\left[\begin{matrix} cosx = sin \dfrac{\pi}{7}\\ cosx = -sin\dfrac{\pi}{7}\end{matrix}\right.$

`<=>` $\left[\begin{matrix} cosx = sin \dfrac{\pi}{7}\\ cosx = sin\dfrac{-\pi}{7}\end{matrix}\right.$

`<=>` $\left[\begin{matrix} cosx = cos(\dfrac{\pi}{2} - \dfrac{\pi}{7})\\ cosx = cos(\dfrac{\pi}{2} + \dfrac{\pi}{7})\end{matrix}\right.$

`<=>` $\left[\begin{matrix}x= \dfrac{5\pi}{14} + k2\pi\\x = \dfrac{-5\pi}{14} + k2\pi\\x = \dfrac{9\pi}{14} + k2\pi\\x = \dfrac{-9\pi}{14} + k2\pi\end{matrix}\right.$ `(k \in ZZ)`

Vậy `S= {+-(5\pi)/(14) + k2\pi; +- (9\pi)/(14) + k2\pi} (k \in ZZ)`

`***`sharksosad