## College Algebra (10th Edition)

$x^4\lt 9x^2\\x^4-9x^2\lt0\\x^2(x^2-9)\lt0$ We know that $(x-k)^2\geq0$ for all $k$. So here for the inequality to be true, $x^2-9\lt0\\x^2\lt9$ $x^2\lt9$ when $-3\lt x\lt3$ So the solution is $-3\lt x\lt3$