Asked by Taosui Li
on 20 Aug 2019 at 20:57

Hello everybody,

I have a little trouble here. I trying to graph/plot the following equation with MATLAB. I already solved the integral of this equation:

f(t)=0.111627907+∑_(n=1)^∞▒〖(1/430 ∫_0^50▒〖((-0.000003072(t-25)^4+1.2) cos(2nπt/860) )dt(nπt/430))〗〗+∑_(n=1)^∞▒〖(1/430 ∫_0^50▒〖((-0.000003072(t-25)^4+1.2 ) sin(2nπt/860) )dt(nπt/430))〗〗

f(t)=0.111627907+∑_(n=1)^∞▒(1/430 1/((π)^4 n^4 ) (-177504/5 (π)^2 n^2+7876917504/125)+1(π)^5 n^5 (-177504/625 (-6450(π)^2 n^2+3816336) sin(5/43 πn)-17750462/5 (125(π)^3 n^3-221880πn) cos(5/43 πn) )(nπt/430)) +∑_(n=1)^∞▒〖(1/430 1/((π)^5 n^5 )(45796032/25 (π)^2 n^2-677414905344/625)+1/((π)^5 n^5 )(-177504/625 (6450(π)^2 n^2-3816336)cos(5/43 πn)-177504/625 (125(π)^3 n^3-221880πn)sin(5/43 πn)(nπt/430))〗

Answer by Walter Roberson
on 20 Aug 2019 at 23:34

syms n t

Q = @(v) sym(v); %convert to rational

Pi = sym('pi');

f1 = Q(0.111627907);

f2a = int((Q(-0.3072*10^(-5))*(t - 25)^4 + Q(1.2))*cos((2*n*Pi*t)/860), t, 0, 50)

f2 = symsum(1/430*f2a*n*Pi*t/430, n, 1, inf)

f3a = int((Q(-0.3072*10^(-5))*(t - 25)^4 + Q(1.2))*sin((2*n*Pi*t)/860), t, 0, 50)

f3 = symsum(1/430*f3a*n*Pi*t/430, n, 1, inf)

f(t) = f1 + f2 + f3;

However, f2 returns NaN. Tracing in a different package, the computation does look suspicious.

Walter Roberson
on 21 Aug 2019 at 1:46

Walter Roberson
on 21 Aug 2019 at 19:13

Taosui Li
on 21 Aug 2019 at 19:41

Thank you very much.

I try to do more research on why a negative infinity is returned.

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## Adam Danz (view profile)

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