Construct this surface:

> restart:

> with(plots):

Warning, the name changecoords has been redefined

> with(plottools):

> x:=r*cos(t): y:=r*sin(t):

> r:=0.02*t:

> plot([x,y,t=0..20],scaling=constrained):

> a:=plot3d([s*x,s*y,(1-s)],s=0..1,t=0..50,scaling=constrained,grid=[5,200],color=red):

> vrml(a,"c:/87138/ex10/10-5.wrl",background_color=white):

> display(a);

>