This statement is not inside any function. (It follows the END that terminates the definition of the function "mixing_tank".)

function dy = mixing_tank(t,y,FA0,CA0,FB0,CB0,V)

% Define the model equations

dy = zeros(2,1);

dy(1) = (FA0 * CA0 - (FA0 + FB0) * y(1)) / V;

dy(2) = (FB0 * CB0 - (FA0 + FB0) * y(2)) / V;

end

% Define the initial conditions and parameters

CA0 = 0.1; % mol/L

CB0 = 0.2; % mol/L

FA0 = 100; % L/min

FB0 = 50; % L/min

V = 100; % L

y0 = [CA0, CB0]; % t=0

% Define the time interval

tspan = [0, 30*60]; % 30 hours

% Solve the ODEs

[t,y] = ode45(@(t,y) mixing_tank(t, y, FA0, CA0, FB0, CB0, V), tspan, y0);

% Plot the results

figure

plot(t, y(:,1), '-', t, y(:,2), '--');

xlabel('Time (min)');

ylabel('Concentration (mol/L)');

legend('C_A', 'C_B');

Answer 1

[Stephen23]

The function definition needs to come after all of the other code. See:

https://www.mathworks.com/help/matlab/matlab_prog/local-functions-in-scripts.html

% Define the initial conditions and parameters
CA0 = 0.1; % mol/L
CB0 = 0.2; % mol/L
FA0 = 100; % L/min
FB0 = 50; % L/min
V = 100; % L
y0 = [CA0, CB0]; % t=0
% Define the time interval
tspan = [0, 30*60]; % 30 hours
% Solve the ODEs
[t,y] = ode45(@(t,y) mixing_tank(t, y, FA0, CA0, FB0, CB0, V), tspan, y0);
% Plot the results
figure
plot(t, y(:,1), '-', t, y(:,2), '--');
xlabel('Time (min)');
ylabel('Concentration (mol/L)');
legend('C_A', 'C_B');

function dy = mixing_tank(t,y,FA0,CA0,FB0,CB0,V)
% Define the model equations
dy = zeros(2,1);
dy(1) = (FA0 * CA0 - (FA0 + FB0) * y(1)) / V;
dy(2) = (FB0 * CB0 - (FA0 + FB0) * y(2)) / V;
end