documentclass[border=15pt,pstricks]{standalone}
usepackage{pst-plot}
begin{document}
begin{pspicture}[showgrid,algebraic](-7,-1)(7,7)
% uput[90](1,3){ $sqrt{40-x^2}$} % Line 1
% $sqrt{40-x^2}$ % Line 2
psplot[plotpoints=500]{ 40 sqrt neg}{40 sqrt}{sqrt(40-x^2)}
end{pspicture}
end{document}