[tex]\bf \textit{equation of a circle}\\\\
(x- h)^2+(y- k)^2= r^2
\qquad
center~~(\stackrel{}{ h},\stackrel{}{ k})\qquad \qquad
radius=\stackrel{}{ r}\\\\
-------------------------------\\\\
(x- \stackrel{h}{2})^2+(y- \stackrel{k}{4})^2= 9\implies (x- \stackrel{h}{2})^2+(y- \stackrel{k}{4})^2= \stackrel{r}{3^2}
~~
\begin{cases}
center~(2,4)\\
radius=3
\end{cases}[/tex]
