# differentiating a fit formula

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Josh on 29 Nov 2022
Commented: Torsten on 30 Nov 2022
I wish to extract the fit formula or function along with its coefficients, then differentiate it to get its derivative function and also integrate the fit function to get its integration. How to do that please suggest.
x=dataset(:,1);y=dataset(:,2);
[fit, gof] = createFit(x, y)
fx = differentiate(fit,x);
df_fit=(x.^2)*3+2*x.+1;
plot(x,[fx-dif_fit])
function [fitresult, gof] = createFit(x, y)
% Fit: 'untitled fit 1'.
[xData, yData] = prepareCurveData( x, y );
% Set up fittype and options.
ft = fittype( 'poly3' );
% Fit model to data.
[fitresult, gof] = fit( xData, yData, ft );
end

Walter Roberson on 29 Nov 2022
differentiate is specifically designed to differentiate cfit and sfit objects.
integrate is designed to integrate cfit objects.
##### 3 CommentsShow 1 older commentHide 1 older comment
Walter Roberson on 30 Nov 2022
df_fit=(x.^2)*3+2*x.+1; is the derivative of x^3 + x^2 + x + const but poly3 is going to create a model of the form a*x^3 + b*x^2 + c*x + const which would have a derivative of 3*a*x^2 + 2*b*x + c
Torsten on 30 Nov 2022
Isn't the derivative with respect to the fitting parameters required here ? [x.^3,x.^2,x] ?

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