%21. session example 1

[X,Y] = meshgrid(-3:.2:3, -3:.2:3);
Z = Y.^2-X.^2;

figure(1)
contour(X,Y,Z,16);

figure(2)
surfc(X,Y,Z)

figure(3)
surfl(X,Y,Z);
shading interp;
colormap(gray);

figure(4)
mesh(X,Y,Z)
