Question 699419
{{{f(x) = x^4+x^2-20}}}......replace {{{x^2}}} with {{{5x^2-4x^2}}}

{{{f(x) = x^4+5x^2-4x^2-20}}}..group

{{{f(x) = (x^4+5x^2)-(4x^2+20)}}}

{{{f(x) = x^2(x^2+5)-4(x^2+5)}}}

{{{f(x) = (x^2-4)  (x^2+5)}}}

{{{f(x) = (x-2) (x+2) (x^2+5)}}}

solutions:

{{{ (x-2) (x+2) (x^2+5)=0}}}

if {{{ x-2=0}}}...=>...{{{ x=2}}}

if {{{ x+2=0}}}...=>...{{{ x=-2}}}

if {{{ x^2+5=0}}}...=>...{{{ x^2=-5}}}...=>...{{{ x=sqrt(-5)}}}...=>...{{{ x=2.24i}}} or {{{ x=-2.24i}}}


 {{{ graph( 600, 600, -5, 5, -30, 10, x^4+x^2-20) }}}