Noting that the polynomial is even, the roots being
x=\cos{\frac{(2k + 1)\pi}{12}}\ \text{for}\ k\in\{0, 1, 2, 3, 4, 5\}
are more conveniently taken as
x=\cos{\frac{(2k + 1)\pi}{12}}\ \text{for}\ k\in\{-3, -2, -1, 0, 1, 2\}
and hence are
x=\pm\cos{\frac{\pi}{12}},\ \pm\cos{\frac{\pi}{4}} =...