## Calculus 10th Edition

$f(x)$ has an inverse function on the given interval.
We find the derivative of $f(x)$: $f'(x) = -\frac{8}{x^3}$ When the domain is from $(0, \infty)$, $x^3$ is always positive so $f'(x)$ is always negative. Therefore, $f(x)$ is strictly monotonic.