Find the interval(s) on which $f(x) = x + \frac{4}{x}$ is strictly decreasing.

Explanation

1. 01

   $f'(x) = 1 - \frac{4}{x^2} = \frac{x^2 - 4}{x^2}$, defined for $x \ne 0$.

2. 02

   It is negative for $0 < \lvert x \rvert < 2$: $(-2, 0) \cup (0, 2)$.