Fsolve or solve function problem

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David André Gomez Romero 2019-6-27

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https://ww2.mathworks.cn/matlabcentral/answers/469154-fsolve-or-solve-function-problem

评论： David André Gomez Romero 2019-6-28

Hello,

I want to resolve a function with fsolve which my only variable is XX :

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FUN = @(XX) Force_SE(XX, LTendon_slack_i, L_totale, Angle_Pen_i) - Force_PE(XX,F_iso_max_i, L_iso)*cos(Angle_Pen_i);

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This function containt two other function (Force_SE, Force_PE) which are constraint according to the value of XX, for exemple in Force_SE (parameters like eps_m0, k_PE, .. are double value defined previously) :

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Fiber_length_normalized = XX/L_iso;

if (Fiber_length_normalized > (1 + eps_m0) )

F_passive_i = F_iso_max_i*(1 + (k_PE/eps_m0)*(Fiber_length_normalized - (1+eps_m0)));

elseif (Fiber_length_normalized <= (1 + eps_m0) )

F_passive_i = F_iso_max_i*exp(k_PE*(Fiber_length_normalized - 1)/eps_m0)/exp(k_PE);

else

fprintf('Problem active Force')

end

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The problem is that Force_PE contains a symbolique value a_i (F_passive_i, F_iso_max_i, gamma, are double value previously defined)

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cst = exp(-(Fiber_length_normalized-1)^2/gamma);

syms a_i;

F_Active_i = a_i*F_iso_max_i*cst;

F_Paral_elem = F_passive_i + F_Active_i;

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If I understand well FSOLVE will never give me a solution containing a sym value. Then I was thinking to resolve it with the function SOLVE but with this function how the constraint will be respected ? If we use SOLVE, my variable is a sym-value, then in the IF-loop of the constraint an error will appear because we can not compare a double to a sym.... I have the code well define if asked I can send it.

Thanks in advance for the help.

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David André Gomez Romero 2019-6-27

Matt J, you are completelty right, I can use instead of a sym also a regular variable. The only thing, it is that at this moment of the code I don't want to find a solution for a_i. This variable has to be found quite later in the code with an optimization (FMINCON), this is why I wanted to put a symbolique value.

In fact, it would be great if I can obtain a function for XX depending of 'a_i' --> XX(a_i). And that's why I want use the function SOLVE found an expression which i can fit in my optimization inovlinvg FMINCON. But still, it remains some problems :

I really don't know how to write XX as a function of 'a_i' using for exemple SOLVE due to the fact that we have in the expression of Force_SE and Force_PE condition on XX. (if i did not have some constraint on the function the problem will be quite easy and could use simply SOLVE to find an expression of XX function of 'a_i' which I can use later on the FMINCON optimization as another constraint).

Thanks for the answer.

David André Gomez Romero 2019-6-28