How to draw D in 3D ?

D defined by x^2+y^2+z^2=4 and x+y+z=0

Answer 1

[KSSV]

x = linspace(0,1) ;
y = linspace(0,1) ;
[X,Y] = meshgrid(x,y) ;
D1 = (4-X.^2+Y.^2).^0.5 ;
figure
surf(X,Y,D1) ;

% x+y+z=0
D2 = -(X+Y) ;
figure
surf(X,Y,D2)

Answer 2

[Carlos Guerrero García]

There is a wrong sign in the KSVV isolation of z in the first equality, and so, the D1 definition must be

D1=(4-X^2-Y^2).^0.5

Also, I think that Tran Phuc question is about a joint representation of D1, D2, and the curve intersecction of D1 and D2, and so, I suggest the following code:

[x,y]=meshgrid(-2:0.1:2);
z1=-(x+y);
surf(x,y,z1); %The plane
hold on;
[r,t]=meshgrid(0:0.1:2,0:pi/60:2*pi); % Polar coordinates are nice for sphere representations
x=r.*cos(t);
y=r.*sin(t);
z2=sqrt(4-r.^2);
surf(x,y,z2); % The upper semisphere
surf(x,y,-z2); % The lower semisphere
theta=0:pi/60:2*pi;
ro=sqrt(2./(1+sin(theta).*cos(theta))); % After solving the system by hand
x=ro.*cos(theta); % The classical conversion
y=ro.*sin(theta); % The classical conversion
plot3(x,y,-(x+y),'or','MarkerSize',3);
axis equal