Question 848794
{{{x^4- 9x^2 +20=0}}}
<P>
Substitute {{{u=x^2}}}<P>
{{{u^2 -9u + 20 = 0}}}
(u-4)(u-5) = 0<P>
u = 4 or 5<P>
{{{x^2 = 4}}} so x = + or - 2<P>
{{{x^2 = 5}}} so x = + or - {{{sqrt(5)}}}