Why this equation cannot be fitted?

I'm trying to fit the function below:

fitfunction = strcat('l*x*z+l*y*z');
myfittype = fittype(fitfunction,...
'dependent',{'f'},'independent',{'x','y','z'},...
'coefficients',{'l'});

However, Matlab wouldn't allow me to creat the fittype. The error message here:

"Expression l*x*z+l*y*z is not a valid MATLAB expression, has non-scalar coefficients, or cannot be evaluated:

Not enough inputs to FITTYPE function."

Does anybody know why this happened? Thanks!

Answer 1

[Walter Roberson]

You cannot use 3 or more independent variables.

f = l*(x+y)*z

Implies

f/(z*(x+y)) = l

Which becomes the defining estimate with the understanding that / is the matlab matrix division operation