math question

[Manolo Sobrino]

Why don't you try a parametric plot?

Mode->Par
\begin{equation}
X_{1T} = h + r*cos(T)
\end{equation}
\begin{equation}
Y_{1T} = k + r*sin(T)
\end{equation}
Zoom->ZSquare or ZDecimal or ZInteger

If you insist on using Cartesian coordinates, then

Mode->Func
\begin{equation}
Y_{1} = k +\sqrt{r^2-(x-h)^2}
\end{equation}
\begin{equation}
Y_{2} = k -\sqrt{r^2-(x-h)^2}
\end{equation}
Zoom->ZSquare or ZDecimal or ZInteger