How to fit this eqation:y=a-b+*c^x ?

Hi,

I am wondering how can I fit the following equation: y=a-b+*c^x since it is not included in the library and how can I exclude some point without using the curve fitting toolbox. When I use the following expression,I receive an error message

g = fittype('a-b*c^x','coeff',{'a','b','c'})

[curve2,gof2] = fit(y2,Av,g)

Thank you in advance

[John D'Errico]

You help those who would answer you if you show the error message that you get! Show the COMPLETE error message.

[afrya]

yes of course,the error message:

Warning: Start point not provided, choosing random start point. > In Warning>Warning.throw at 31 In fit>iFit at 320 In fit at 108 In projecttest6 at 155

[afrya]

Error using fittype>iInvalidParameterError (line 1045) The parameter "Exclude" is not valid.

Error in fittype>iEnsureValidParameterName (line 1175) iInvalidParameterError( parameter );

Error in fittype>iParseParameters (line 544) parameter = iEnsureValidParameterName( parameter );

Error in fittype>iCreateCustomFittype (line 424) obj = iParseParameters(obj,varargin(2:end));

Error in fittype>iCreateFittype (line 348) obj = iCreateCustomFittype( obj, varargin{:} );

Error in fittype (line 325) obj = iCreateFittype( obj, varargin{:} );

Error in projecttest6 (line 154) g = fittype('a-b*c^x','coeff',{'a','b','c'},'Exclude',exclude1)

[Image Analyst]

Is b a scalar constant, like a, or is it a coefficient like c? You've done it both ways.

[afrya]

a, b, and c are all coefficients

[Image Analyst]

Looking more closely I think that you meant b to be a multiplicative factor, not a scalar that you subtract and that you just made a mistake when you wrote "y=a-b+*c^x". Because of what Roger said, and you said later, I think what you really meant was y=a-b*c^x and you just put in the + sign by mistake.

Answer 1

[Roger Stafford]

Instead of 'a-b*c^x' you might try the equivalent 'a-b*exp(k*x)'. If you can fit to that, then c = exp(k) will be a fit for 'a-b*c^x'.