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Error in solving ode using symbolic calculations

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Roman Gilgor
Roman Gilgor on 1 Mar 2020
Edited: Roman Gilgor on 1 Mar 2020
hi, I am trying to compare my solution using ode45 and runge kutta to analytical solution of symbolic matlab.
Iam solving a simple system of linear ode equations: (this is the code)
syms s1(t) s2(t) I(t) u(t)
u(t) = 10; % input
% system of odes
ode1 = diff(s1) == u(t) - s1/55; % equation 1
ode2 = diff(s2) == s1/55 - s2/55; % equation 2
ode3 = diff(I) == s2/(0.12*55*60) - 0.138*I; % equation 3
odes = [ode1; ode2; ode3];
%initial conditions
cond1 = s1(0) == 0;
cond2 = s2(0) == 0;
cond3 = I(0) == 0;
conds = [cond1; cond2; cond3];
[s1Sol(t), s2Sol(t),Isol(t)] = dsolve(odes,conds)
I would like to see what Isol(t) is equal to. Every time I change the numbers at equation 2 or 3 it changes the solution of only equation 1! why????
I solved this analyticly on papper and its definetly wrong.
both ode45 and runge kutta gives me the same answer but for some reason analytical matlab solution is wrong.
Please help.
Thanks allot!!!
by the way this is ode45 function
% initial conditions are 0 0 0
% time interval 0 - 10
function dydt = odefun(t,y)
dydt = zeros(3,1);
dydt(1) = 10 - y(1)/55;
dydt(2) = y(1)/55 - y(2)/55;
dydt(3) = y(2)/(55*0.12*B60) - 0.138*y(3);
end

  2 Comments

Roman Gilgor
Roman Gilgor on 1 Mar 2020
Hi, avoid this ita my mistake in copying it from matlab. It just 0.12*60 Thanks!

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