Trigonometry+Polynomials=Trigonomials

Geometry Level 5

$\large 8x^3-6x+1 = 0$

If $a$ , $b$ and $c$ are the roots of the equation above, find the value of $\cos^{-1} a + \cos^{-1} b + \cos^{-1} c$ in degrees.

If you think that roots exist outside $[-1, 1]$ , write 272 as your answer.