[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-5)^2+y^2=81\implies (x-\stackrel{h}{5})^2+(y-\stackrel{k}{0})^2=\stackrel{r}{9^2}~~
\begin{cases}
center~(5,0)\\
radius=9
\end{cases}[/tex]
