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How to solve differential equations which has more time dependent arguments ?

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pritesh patel 2021-5-28

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编辑： Torsten 2021-5-29

I have equation gdot=-k*g+xdot, where xdot=-k*x+sin(t). how can I solve gdot using ode45 ?

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Torsten 2021-5-28

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Insert the expression from the second equation for xdot in the first equation and solve both equations simultaneously.

I think you can use dsolve instead of ode45 since the system most probably has an analytical solution.

J. Alex Lee 2021-5-29

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if you use ode45, you don't need to insert the expression for xdot into the first equation...did you read documentation for ode45, and did you try to write an odefun yourself?

Torsten 2021-5-29

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If you use the mass matrix option, you don't need to insert the expression for xdot into the first equation.

If you don't use the mass matrix option, you must.

J. Alex Lee 2021-5-29

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@Torsten, interesting about the mass matrix method...but without it, I am just saying there wouldn't be a need to duplicate the expression from a coding perspective, right?

Torsten 2021-5-29

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编辑：Torsten 2021-5-29

Of course, you can write

function dy = fun(t,y)

k = ...;

dy = zeros(2,1)

dy(1) = -k*y(1) + sin(t); % equation for x

dy(2) = -k*y(2) + dy(1) % equation for g

end

Thus you don't need to repeat the formula for dy(1) in dy(2), if this is what you meant to ask.

J. Alex Lee 2021-5-29

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haha yes exactly...but now the answer is given away :)

and g should likely be Y(2) as far as we know from pritesh's nomenclature

Torsten 2021-5-29

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