$$x^6+104x^4+400x^2+1 = 0$$
Answer
$$ \begin{matrix}x_1 = 0.05002i & x_2 = -0.05002i & x_3 = 1.99935i \\[1 em] x_4 = -1.99935i & x_5 = 10.00001i & x_6 = -10.00001i \end{matrix} $$
Explanation
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using Newton method.
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Polynomial Equations Solver