# Equation of a sigmoid

Marco Atzori shared this question 3 years ago

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?

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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

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try FitLogistic( <List of Points> )

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Can I take the List of Points from a spreadsheet? The column "A" is the x coordinates, the column "B" is the y coordinate

https://www.geogebra.org/cl...

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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

perhaps a traslation of points gives better result