clc, clear all, close all
f = @(x, y) sqrt(9 - x.^2 - y.^2);
fsurf(f, [-4 4 -4 4])
xlabel('x')
ylabel('y')
zlabel('z')
%hold on
%f = @(x, y) -sqrt(9 - x.^2 - y.^2);
%fsurf(f, [-4 4 -4 4])