Question 16274
Solve:

{{{x^4 - 10x^2 + 9 = 0}}} This can be factored by treating the x^4 as (x^2)^2.
{{{(x^2)^2 - 10(x^2) + 9 = 0}}}
{{{(x^2 - 1)(x^2 - 9) = 0}}} Apply the zero products principle.
{{{x^2 - 1 = 0}}} and/or {{{x^2 - 9 = 0}}} 

If {{{x^2 - 1 = 0}}}, then {{{x^2 = 1}}} and {{{x = +1}}} or {{{x = -1}}}
If {{{x^2 - 9 = 0}}}, then {{{x^2 = 9}}} and {{{x = 3}}} or {{{x = -3}}}

The four roots are:
x = -1
x = 1
x = -3
x = 3

So, in increasing order, we have:

x1 = -3
x2 = -1
x3 = 1
x4 = 3