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How can I calculate the slope of third order polynomial fit

Asked by Lameck Amugongo on 12 Jul 2019
Latest activity Commented on by Lameck Amugongo on 12 Jul 2019
I have a third order polynomial fit. I would like to calculate the slope of the fit. Any ideas on how i can do this? Is possible to obtain this from the coefficients?
[p,S] = polyfit(x,y,3);
[y_fit,delta] = polyval(p,x,S);

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Answer by John D'Errico
on 12 Jul 2019

Is there a reason why you would not think to differentiate it?
help polyder

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I would just want to see if the measurement is decreasing/increasing/remaining constant overtime.
for example, [p,S] = polyfit(x,y,1);
p(1) will give the slope of the fit.
How can i get the same slope from high order fit e.g. [p,S] = polyfit(x,y,3); or [p,S] = polyfit(x,y,2);?
Thanks

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