[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^2+y^2=42\implies (x-\stackrel{h}{0})^2+(y-\stackrel{k}{0})=(\stackrel{r}{\sqrt{42}})^2~~
\begin{cases}
center~(0,0)\\
radius=\sqrt{42}
\end{cases}[/tex]
