Editing Newton's method (section)

===Difficulty in calculating the derivative of a function===
Newton's method requires that the derivative can be calculated directly. An analytical expression for the derivative may not be easily obtainable or could be expensive to evaluate. In these situations, it may be appropriate to approximate the derivative by using the slope of a line through two nearby points on the function. Using this approximation would result in something like the [[secant method]] whose convergence is slower than that of Newton's method.