Difference- and differential quotient

In this applet you can see the function f: R → R; f(x) = 0,1·x² + 2 Move point B using the slider for Δx towards point A. Take a note for the slope k of the secant through point A and B for Δx = 3,5 ; 3,0 ; 2,5; 2,0; 1,5; 1,2 and 1,1. Which value could be assumed for the slope of the tangent in point A as a limit of the slopes for the secants? Repeat the exercise for function f(x) = 0,1·x².
Andreas Lindner