Evaluate: x^4-5x^2+4=0

Expression: ${x}^{4}-5{x}^{2}+4=0$

Transform the biquadratic equation into a quadratic equation by substituting $t$ for ${x}^{2}$

${t}^{2}-5t+4=0$

Solve the equation for $t$

$\begin{array} { l }t=1,\\t=4\end{array}$

Substitute back $t={x}^{2}$

$\begin{array} { l }{x}^{2}=1,\\{x}^{2}=4\end{array}$

Solve the equation for $x$

$\begin{array} { l }x=-1,\\x=1,\\{x}^{2}=4\end{array}$

Solve the equation for $x$

$\begin{array} { l }x=-1,\\x=1,\\x=-2,\\x=2\end{array}$

The equation has $4$ solutions

$\begin{array} { l }x_1=-2,& x_2=-1,& x_3=1,& x_4=2\end{array}$