definite integral not giving exact answer

8 views (last 30 days)
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

Accepted Answer

Steven Lord
Steven Lord on 15 Jan 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])

More Answers (1)

Cameron B
Cameron B on 15 Jan 2020
Edited: Cameron B on 15 Jan 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)))

Products

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!