How to resolve this error "Unrecognize function or variable" when using fsolve

I am trying to solve a system of two equations with two varaibles using fsolve in live script. What primarily I want to do is to plot the suface of the output. Below are the

codes for the two scripts I used

First script:

function F = myfun(x)
F = nan(1,2)
F(1) = cosh(b.*a.^2*x(1)^2 + b.*(1-a).^2*x(2)^2)-x(1);
F(2) = cosh(2*b.*a.*(1-a)*x(1)*x(2) + b*.(1-a).^2*x(2)^2)-x(2);
end

Second script;

b = linspace(-2,2,100);
a = linspace(0,1,100);

x0 = [0,0];

x = fsolve(@(x)myfun(x),x0)
Z = a.*x(1) + (1-a).*x(2)

figure
[X,Y] = meshgrid(b,a);
surf(Y,X,Z')

Edit: This is the error message I get

Unrecognized function or variable 'b'.

Error in myfun (line 3)

F(1) = cosh(b*a^2*x(1)^2 + b*(1-a)^2*x(2)^2)-x(1);

Error in myfunTanh (line 9)

x = fsolve(@(x)myfun(x),x0)

Error in fsolve (line 260)

fuser = feval(funfcn{3},x,varargin{:});

Caused by:

Failure in initial objective function evaluation. FSOLVE cannot continue.

Answer 1

[Matt J]

We need to see the full error message copy/pasted, but you may have put the myfun() script in a location where Matlab cannot see it.

Also, you are not passing a and b to myfun(), so myfun() sees them as undefined.

Also, a and b are vectors in your second script, but treated as scalars within myfun, so you need to resolve that as well.

[Godwin Osabutey]

After is I made it element wise operation and included it 'a' and 'b' in the myfun(), I got this error;

Unable to perform assignment because the left and right sides have a different number of elements.

Error in myfun (line 3)

F(1) = cosh(b.*a.^2*x(1)^2 + b.*(1-a).^2*x(2)^2)-x(1);

Error in myfunCosh (line 9)

x = fsolve(@(x)myfun(x,a,b),x0)

Error in fsolve (line 260)

fuser = feval(funfcn{3},x,varargin{:});

Caused by:

Failure in initial objective function evaluation. FSOLVE cannot continue.

[Torsten]

You assign a vector of 100 values (cosh(...)) to one scalar element (F(1)).

Same for F(2).

So you will have to call fsolve in a loop 100 times instead.

[Matt J]

"Also, a and b are vectors in your second script, but treated as scalars within myfun, so you need to resolve that as well."