Answer
$a\lt0$
Work Step by Step
$\bf{\text{Solution Outline:}}$
Use the concept of inequality to translate the given description,
\begin{array}{l}\require{cancel}
\text{a number $a$ is negative}
,\end{array}
to symbols.
$\bf{\text{Solution Details:}}$
Negative numbers are numbers less than $0.$ Hence, in symbols, the given description is equivalent to
\begin{array}{l}\require{cancel}
a\lt0
.\end{array}
