$$16x^4x-81 = 0$$
Answer
$$ \begin{matrix}x_1 = 1.38316 & x_2 = -1.119+0.813i & x_3 = -1.119-0.813i \\[1 em] x_4 = 0.42742+1.31547i & x_5 = 0.42742-1.31547i \\[1 em] \end{matrix} $$
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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