8x^3 -4x^2 +2x+1 = 0

`8x^3 + 4x^2 + 2x + 1 = 0`

`<=>` `(8x^3 + 4x^2) + (2x + 1) = 0`

`<=>` `4x^2(2x + 1) + (2x + 1) = 0`

`<=>` `(2x + 1)(4x^2 + 1) = 0`

     `=>` $\left[\begin{matrix} 2x + 1 = 0\\ 4x^2 + 1 = 0\end{matrix}\right.$

      `=>` $\left[\begin{matrix} x = -1/2 (TM)\\ 4x^2 = -1 (loại)\end{matrix}\right.$

      Vậy `x = -1/2`