definite integral not giving exact answer

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AgentWindu
AgentWindu el 15 de En. de 2020
Comentada: AgentWindu el 16 de En. de 2020
Hi,
I have a triple integral with the following code:
inner_int=int(r*sin(phi),phi,[acos(4/5) pi-acos(4/5)])
middle_int=int(inner_int,r,3,5)
Mass_S=int(middle_int,theta,0,2*pi)
The answer I get from the first line is
-r*(cos(175787564848171/70368744177664) - cos(181129459610871/281474976710656))
When it should be 8/5*r
I have tried adding 'IgnoreAnalyticConstraints',true to the end of int but it didn't work. Is there anything I can do to get MATLAB to compute this as an exact solution. For reference the shape to be integrated is a sphere with a cylindircal hole drilled in it and spherical coordinates are required. Thanks

Respuesta aceptada

Steven Lord
Steven Lord el 15 de En. de 2020
Compute your limits of integration symbolically to avoid roundoff error.
f = sym(4)/5;
a = acos(f);
inner_int = int(r*sin(phi), phi, [a, pi-a])

Más respuestas (1)

Cameron B
Cameron B el 15 de En. de 2020
Editada: Cameron B el 15 de En. de 2020
I don't know what the issue is. I did this and got 8*r/5. The answer seems to be correct.
syms r
sym(-r*(cos(175787564848171/70368744177664) - cos(181129459610871/281474976710656)))

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