$$(5x^2-4)^2 = 0$$
Answer
$$ \begin{matrix}x_1 = 0.89443 & x_2 = -0.89443 \\[1 em] \end{matrix} $$
Explanation
$$ \begin{aligned} (5x^2-4)^2 &= 0&& \text{simplify left side} \\[1 em]25x^4-40x^2+16 &= 0&& \\[1 em] \end{aligned} $$
This polynomial has no rational roots that can be found using Rational Root Test.
Roots were found using quartic formulas
This page was created using
Polynomial Equations Solver