Inequality

Posted May 6th, 2009 by alpha

$x^4-2x^2-63\le0$

Comments

On May 6th, 2009 alpha says:

I'm getting $x\in[-3,\sqrt{7}] U [\sqrt{7},3]$
but the answer given is $x\in[-3,3]$

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On May 8th, 2009 Structure says:

$x^4-2x^2-63\le0$
$x^4-2x^2-63=(x^2-9)(x^2+7)=(x-3)(x+3)(x^2+7)\le0$

$x^2+7=0$

has complex solutions $x=\pm i\sqrt 7$ and is strict positive on real numbers so has no influence.
So

$x\in[-3,3]$