## Calculus (3rd Edition)

$(-\infty,0)$.
For the function $f(x)=\frac{1}{x^4+x^2+1}$, we note that the denominator is positive for all values of $x$. Since the decreasing of the absolute value of x make the denominator decrease, the function $f(x)$ is increasing. So the function in increasing on the interval $(-\infty,0)$.