> N[2]:=xi*(3*xi-1)*(3*xi-2)/2: N[3]:=eta*(3*eta-1)*(3*eta-2)/2:
> N[4]:=(1-xi-eta)*(3*(1-xi-eta)-1)*(3*(1-xi-eta)-2)/2:
> N[7]:=9*(3*xi-1)*xi*eta/2: N[8]:=9*(3*eta-1)*xi*eta/2:
> N[13]:=9*(3*xi-1)*xi*(1-xi-eta)/2:
> N[14]:=9*(3*(1-xi-eta)-1)*xi*(1-xi-eta)/2:
> N[15]:=9*(3*eta-1)*eta*(1-xi-eta)/2:
> N[16]:=9*(3*(1-xi-eta)-1)*eta*(1-xi-eta)/2:
> N[19]:=27*xi*eta*(1-xi-eta):
> x:=N[2]+(N[7]+N[13])*cos(Pi/6)+(N[8]+N[14])*cos(Pi/3)+N[19]/sqrt(3):
> x:=expand(%):
> x:=simplify(%):
> x:=factor(%):
> y:=N[3]+N[7]*sin(Pi/6)+N[8]*sin(Pi/3)+N[15]*cos(Pi/6)+N[16]*cos(Pi/3)+
> N[19]/sqrt(3):
> y:=expand(%):
> y:=simplify(%):
> y:=factor(%):
> z:=N[4]+(N[13]+N[15])*sin(Pi/6)+(N[14]+N[16])*sin(Pi/3)+N[19]/sqrt(3):
> z:=expand(%):
> z:=simplify(%):
> z:=factor(%):
> plot1:=plot3d([x,y,z],xi=0..1,eta=0..1-xi,axes=normal,orientation=[-60
> ,80],tickmarks=[2,2,2],shading=zgrayscale):
> plot2:=plot3d(1-xi-eta,xi=0..1,eta=0..1,view=[0..1,0..1,0..1],style=wi
> reframe,color=black,tickmarks=[2,2,2]):
> plots[display]({plot1,plot2});

> rr:=x^2+y^2+z^2:
> rr:=expand(%):
> rr:=simplify(%):
> r:=+sqrt(%):
> L_[2]:=evalf(sqrt(int(int((1-r)^2,eta=0..1-xi),xi=0..1)));

                       L_[2] := 0.001185186852

> l_[2]:=evalf(sqrt(int(int((1-r^2)^2,eta=0..1-xi),xi=0..1)));

                       l_[2] := 0.002368590675

> delta:=(1-r^2)^2-(1-r)^2:
> plot3d(1-r,xi=0..1,eta=0..1-xi,axes=normal,orientation=[-60,80],tickma
> rks=[3,3,3]);

> 
