Chapter 4 - Applications of the Derivative - 4.2 What Derivatives Tell Us - 4.2 Exercises - Page 256: 18

$f$ is decreasing on $(-\infty,0)$, $f$ is increasing on $(0,\infty)$.

$f'(x) = 2x$, which is zero exactly when $x = 0$. On $(−∞, 0)$ we note that $f' < 0$, so that $f$ is decreasing on this interval. On $(0,∞)$, we note that $f' > 0$, so $f$ is increasing on this interval.