#### Answer

In converting $r=5$ from a polar equation to a rectangular equation, square both sides of the equation such that ${{r}^{2}}$ gets replaced with ${{x}^{2}}+{{y}^{2}}$.

#### Work Step by Step

Consider the given polar equation, $r=5$
Squaring both sides of the equation we get,
$\begin{align}
& {{r}^{2}}={{5}^{2}} \\
& =25
\end{align}$
Use, ${{r}^{2}}={{x}^{2}}+{{y}^{2}}$
Therefore,
${{x}^{2}}+{{y}^{2}}=25$
Hence, to convert $r=5$ from a polar equation to a rectangular equation, we will square both sides of the equation such that ${{r}^{2}}$ gets replaced with ${{x}^{2}}+{{y}^{2}}$.