definite integral not giving exact answer

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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

Risposta accettata

Steven Lord
Steven Lord il 15 Gen 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])

Più risposte (1)

Cameron B
Cameron B il 15 Gen 2020
Modificato: Cameron B il 15 Gen 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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