without needing the distance formula
from 4,1 to 0,1, you move the "x" from 4 to 0, over the same "y" value of 1, so is just 4 units long, from the center to an endpoint, and the chord that stems from the center to an endpoint is the radius, so r = 4
[tex]\bf \textit{equation of a circle}\\\\
(x-{{ h}})^2+(y-{{ k}})^2={{ r}}^2
\qquad
\begin{array}{lllll}
center\ (&{{ h}},&{{ k}})\qquad
radius=&{{ r}}\\
&4&1&4
\end{array} [/tex]
