Answer
$\{-2,-1,1,2 \}$
Work Step by Step
$x^{4}-5x^{2}+4 = 0$
$(x^{2})^{2}-5x^{2}+4 = 0$
Substituting $x^{2}=t$
$t^{2}-5t+4 = 0$
By factoring,
$(t-4)(t-1)=0$
$t-4=0$ or $t-1=0$
$t=4$ or $t= 1$
Replacing $t = x^{2}$
$x^{2}=4$ or $x^{2}= 1$
$x=±2$ or $x=±1$
Solution Set is $\{-2,-1,1,2 \}$
