> restart:

> x:=0.5+0.5*cos(t):

> y:=0.5*sin(t):

> z1:=sqrt(1-x^2-y^2):

> z2:=-z1:

> z:=(1-s)*z1+s*z2:

> a:=plot3d ([x,y,z],s=0..1,t=0..2*Pi,color=red,scaling=constrained):

> x:=0.5+r*cos(t):

> y:=r*sin(t):

> z:=sqrt(1-x^2-y^2):

> b:=plot3d ([x,y,z],r=0..1/2,t=0..2*Pi,color=blue,scaling=constrained):

> c:=plot3d ([x,y,-z],r=0..1/2,t=0..2*Pi,color=yellow,scaling=constrained):

> with (plots):

Warning, the name changecoords has been redefined

> display (a,b,c);