clear
a=-2:0.1:2;
b=-3:0.1:3;
[x,y]=meshgrid(a,b);
z=(1-(x.^2)/4-(y.2)/9).^(1/2);
mesh(x,y,z)
hold on
mesh(x,y,-z)

clear
a=-1:0.1:1;
b=-2:0.1:2;
[x,y]=meshgrid(a,b);
z=(4/9)*(x.^2)+(y.2);
mesh(x,y,z)