$$5x^4-15x^2+4 = 0$$
Answer
$$ \begin{matrix}x_1 = 0.54391 & x_2 = -0.54391 & x_3 = -1.64443 \\[1 em] x_4 = 1.64443 & \\[1 em] \end{matrix} $$
Explanation
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
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