> plot3d(x^2-y^2,x=-3..3, y=-4..4);

> plot3d(sin(x/2)*sin(y/2),x=-10..10,y=-10..10);

> with(plots):

> implicitplot3d(x^2+y^2+3*z^2=4,x=-2..2,y=-2..2,z=-2..2);

>