Unrecognized function or variable 'X0'.

Second, you have created a function that appears to be written upside down. It also appears to contain at the very bottom the inputs you are using to call the function in fsolve.

Third, you must be explicit in your use of mathematical operators to use. MATLAB does not assume anything. For example, sigma(X(14) should probably be sigma*(X(14)

Finally, check for typos. For example ((X13) should probably be (X(13)

As for the error you are seeing right now, it is likely because you are trying to call your function without defining the inputs. Here, for example, I haven't defined the variable X0 yet.

[X, FVAL]= FSOLVE(@mintafun,X0,options),
Unrecognized function or variable 'X0'.

The correct syntax might look like this, at which point, you will start getting new errors due to the other issues I describe above.

x0 = [10; 6; 4; 3; 4; 3; 2; 3; 18; 7; 25; 10; 5; 5; 4; 11; 8; 20; 6; 3; 8; 1; 30; 7; 2; 6; 14; 30;]; 
options=optimset('Display','iter'); 
[X, FVAL]= fsolve(@mintafun,x0,options),

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Cris LaPierre 2023-5-27

编辑：Cris LaPierre 2023-5-27

在 MATLAB Online 中打开

I refering to your defining equations that use variables that have not yet been defined. You can't use a variable (or array element, if that is your intent) if it hasn't yet been created.

Here's a shortened example

x0 = [10; 6; 4; 3; 4; 3; 2; 3; 18; 7; 25; 10; 5; 5; 4; 11; 8; 20; 6; 3; 8; 1; 30; 7; 2; 6; 14; 30];
F1 = mintafun(x0)
Unrecognized function or variable 'phi'.

Error in solution>mintafun (line 6)
F =X(5)/X(12) - phi*(1 - tau_c)*X(1) / theta*(1 - tau_w)*(1-X(4));
% The very first calculation uses phi, tau_c, theta, and tau_w, none of which exist yet.
function F = mintafun(X)
F =X(5)/X(12) - phi*(1 - tau_c)*X(1) / theta*(1 - tau_w)*(1-X(4));
tau_c = 0.17;
tau_w = 0.2;
theta = 0.349;
phi = 5;
end