Generally, approximations are best when x is around a, and gets worse the further x gets from a.Taylor Series:About the point x = a:f(x) = f(a) + f'(a) * (x - 1) + f''(a) * (x - a)^2 / 2! + f'''(a) * (x - a)^3 / 3! + ....If the series is cut off at n terms, the final term of the Taylor series is:f^(n+1)(t) / (n+1)! * (x - a)^(n+1).