>>14266043You have two options, the median rate of change of f(x) between two successive x_i and x_j as (f(x_j) - f(x_i))/(j-i), which I suppose is the easy way. In this case, that would be 9/3 = 3.
The hard but better way would be to use an interpolation technique like Lagrange's polynomial interpolation with an appropriate degree of function, usually cubic splines, and then differentiate that function. Fortunately, there's computer programs that will do that for you.
