Figure 2: Two ways of estimating the derivative in point 𝑎 : 𝐷 1 is calculated by symbolically deriving the function in point 𝑎 ; 𝐷 2 is the slope between point 𝑎 and point 𝑏 (where 𝑎 and 𝑏 are calculated from the curve). Dots represent sample points through which a curve is fitted.