/// <summary>
/// Derives f(x)
/// </summary>
/// <param name="f">The function f(x) to be derived</param>
/// <returns>The derivative df/dx as a function</returns>
public static Func<double, double> fprime(Func<double, double> f)
{
	return x =>
	{
		double h = 1;
		double tempf = 0;
		double guessf = 0;
		do
		{
			tempf = guessf;
			guessf = (f(x + h) - f(x)) / h;
			h = h / 10;
		} while (Math.Abs((guessf - tempf) / guessf) > 0.001);

		return guessf;
	};
}