[tex]\bf (x-{{ h}})^2+(y-{{ k}})^2={{ r}}^2
\qquad center\ ({{ h}},{{ k}})\qquad
radius={{ r}}\\\\
-----------------------------\\\\
x^2+y^2=25\implies x^2+y^2=5^2\implies
\begin{array}{llll}
(x-&0)^2+(y-&0)^2=&5^2\\
&\uparrow &\uparrow &\uparrow \\
&h&k&r
\end{array}[/tex]
