def f(x):
    return ((-41*x**4)/12) + ((79*x**3)/6) - ((49*x**2)/12) - ((44*x)/3) + 5
def fderiv(x, deltax):
    return (f(x + deltax) - f(x - deltax)) / 2 * deltax
print fderiv(0, 0.25)
-0.865234375
print fderiv(0, 0.1)
-0.14535