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# How to use function from fitting without manually rewriting it?

Posted 11 years ago
 Hi, I have some data, to which I am fitting a polynomial of degree 4. In[112]:= funkcija = Fit[tocke, {1, x, x^2, x^3, x^4}, x] Out[112]= 7655.48 - 1.37747*10^-16 x - 0.0000580449 x^2 + 1.0626*10^-23 x^3 - 8.35914*10^-13 x^4  Now what I need is actually derivative of this polynomial and I don't want to manually rewrite each coefficient, or is that really necessary? In the end I am interested into values of the derivative in points from my first column of the imported data.
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Posted 11 years ago
 Well now I fell stupid. :DThanks for a nice and quick answer!
Posted 11 years ago
 Dear Mitja,sure, that's quite straight forward. Unfortunately, you did not provide the actual time series so I generated one that might be similar: tocke = Table[{x, 7655.48 - 1.37747*10^-16 x - 0.0000580449 x^2 + 1.0626*10^-23 x^3 - 8.35914*10^-13 x^4 + 1000 RandomVariate[NormalDistribution[]]}, {x, 0, 10000, 100}]; You already fitted a function to that: funkcija = Fit[tocke, {1, x, x^2, x^3, x^4}, x] If you want a derivative you can try: derfunkcija = D[Fit[tocke, {1, x, x^2, x^3, x^4}, x], x] If you want you can evaluate that at all the points of the original time series. Table[D[Fit[tocke, {1, x, x^2, x^3, x^4}, x], x] /. x -> xs, {xs, 0, 10000, 100}] If you prefer taking the x-coordinates directly from the data file you can try this: D[Fit[tocke, {1, x, x^2, x^3, x^4}, x], x] /. x -> # & /@ tocke[[All, 1]] Cheers, Marco