Answer
$-c\lt u\lt c$
Work Step by Step
Step 1. Given $c\gt0$ and $|u|\lt c$, we have, when $u\geq0, |u|=u$, thus $u\lt c$ for this case.
Step 2. When $u\lt0, |u|=-u$, we have $-u\lt c$ and $u\gt -c$ for this case.
Step 3. Combine the above results, we have $-c\lt u\lt c$ for all $u$ values.
