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calculus and analysis - Evaluating a surface integral


I'm trying to compute the integral Sx2+z2dS,

where S is the surface S: x22+y23+z22=1,y0.


One possible parametrization is: x(u,v)=(2cosucosv,3cosusinv,2sinu),

with π2uπ2,0vπ. Then I make:


X[u_,v_] :=  {Sqrt[2] Cos[u] Cos[v], Sqrt[3] Cos[u] Sin[v], Sqrt[2] Sin[u]}


f[{x_, y_, z_}] := x^2 + z^2

Integrate[f[X[u, v]] Norm[Cross[D[X[u, v], u], D[X[u, v], v]]], {u, -π/2, π/2}, {v, 0 , π}]

but Mathematica just won't compute it (keeps running on and on). What is an efficient way to do this? Thanks.



Answer



Solution with NIntegrate


NIntegrate[f[X[u, v]] Norm[Cross[D[X[u, v], u], D[X[u, v], v]]], {u, -π/2, π/2}, {v, 0, π}]
(* 19.8097 *)


An alternative approach to the problem is


s = ImplicitRegion[{x^2/2 + y^2/3 + z^2/2 == 1 && y > 0}, {x, y, z}];
Chop[NIntegrate[x^2 + z^2, {x, y, z} ∈ s], 10^-7]

which, of course, yields the same answer.


Added: Solution with Integrate


Consider the calculation in cylindrical coordinates with the axis along y. Integrating about the axis of symmetry then leaves the integral of 2 π r^3 over the 1D region,


s1 = ImplicitRegion[{r^2/2 + y^2/3 == 1 && r > 0 && y > 0}, {r, y}];
2 π Integrate[r^3, {r, y} ∈ s1]
(* 1/2 π (10 + 3 Sqrt[2] ArcCot[Sqrt[2]]) *)


with the numerical value given earlier.


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