[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} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ (x-\stackrel{h}{3})^2+(y-\stackrel{k}{7})^2=9\implies (x-3)^2+(y-7)^2=3^2~\hfill \boxed{r=3}[/tex]
