Answer
not a real number
Work Step by Step
The given expression, $
\sqrt{16-25}
,$ simplifies to
\begin{array}{l}\require{cancel}
\sqrt{-9}
.\end{array}
Since the index of the given expression, $
\sqrt{-9}
,$ is even (index = $
2
$) and the radicand is negative (radicand = $
-9
$), then the given expression is $\text{
not a real number
.}$