restart;
f:=7+cos(t);
g:=sin(t);
plot([f,g,t=0..2*Pi],scaling=constrained);
plot3d([cos(theta)*f,g,sin(theta)*f],
   t=0..2*Pi,theta=0..2*Pi,scaling=constrained);
V:=2*int(2*Pi*x*sqrt(1-(x-7)^2),x=6..8);
V:=int(2*Pi*x*(4-4*abs(x-7)),x=6..8);