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Using GeoGebra I drew the curve of an acid-base titration. To find the equivalence point, I have to calculate the point where the second derivative is equal to zero. But to do this, I need the equation that describes the sigmoid. With GeoGebra, is it possible to do this?
The same question 
Sorry, what equation did you use to draw? You're the only one who knows it. So if you share your file your problem could be more clear to others too
https://en.wikipedia.org/wi...
try FitLogistic( <List of Points> )
it is easy to create the points in column C writing (A1,B1) then select C1 and drag and drop the little blue square right down of selection rectangle
I did three fits and I think that (x-k)^1/3 is the best but not sigmoid. k is the inflection point
https://www.geogebra.org/cl...
perhaps a traslation of points gives better result
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