take derivative of a function

Hi, I have a function that I defined, and I would like to take the first derivative of this function. I tried different ways. But the program does not work. I must missed something. Below is my code.

%y is independent variable, a and b are parameters;
function [f,c1,c2]=fun(y,a,b)
f=a*log(c1)+b*log(c2);
c1=y*a;
c2=y*b;
end

Then I tried to take the derivative of f with respect to y. And I also want to check the value of c1 and c2 (in terms of y). So I wrote the following code.

[f,c1,c2]=fun(y,a,b);
g=diff(f,y)

I got the following error message

"Undefined function or variable 'y'"

Please help. Thanks.

Answer 1

[Walter Roberson]

syms y

before the [f,c1,c2] assignment.

[Xin CUI]

Thanks. I got g as an expression in terms of y(independent variable) and a,b(parameters). Now the next step is to find the solution of g=0. I wrote the following x=fsolve(g,y0) but got error messages Error using lsqfcnchk (line 108) If FUN is a MATLAB object, it must have an feval method.

Error in fsolve (line 198) funfcn = lsqfcnchk(FUN,'fsolve',length(varargin),funValCheck,gradflag);

Could you see the problem?

[Walter Roberson]

Use solve() rather than fsolve()

Answer 2

[KSSV]

Use symbolic expression:

syms f a b
f = a*log(y*a)+b*log(y*b)
df = diff(f)

[Xin CUI]

Thanks. I got df as an expression in terms of y(independent variable) and a,b(parameters). Now the next step is to find the solution of df=0. I wrote the following x=fsolve(df,y0) but got error messages Error using lsqfcnchk (line 108) If FUN is a MATLAB object, it must have an feval method.

Error in fsolve (line 198) funfcn = lsqfcnchk(FUN,'fsolve',length(varargin),funValCheck,gradflag);

Could you see the problem?

[Xin Zhao]

Thank you!