8-05 Construct one half of a torus so its cross-section consists of two concentric circles

> restart;

> r:=3+cos(t):

> x:=r*cos(s):

> y:=r*sin(s):

> z:=sin(t):

> a:=plot3d([x,y,z],s=0..2*Pi,t=0..Pi,scaling=constrained,grid=[20,30]):

> a;

> with(plottools):

> vrml(a,"c:/8-05.wrl",background_color=white);

>