Question

The solution set of $x^4-8x^2-9\le 0$ is

• A. $(-3,3)$
• B. $(0,3)$
• C. $[-3,3]$
• D. $[3,\infty )$

Answer 1

The correct option is C $[-3,3]$

$x^4-8x^2-9\le 0\Rightarrow (x^2+1)(x^2-9)\le 0\Rightarrow (x+3)(x-3)\le 0$


$\therefore x\in [-3,3]$