double pochodna(double(*pf)(double), double x0)
{
	const double delta = 1.0e-6;
	double x1 = x0 - delta;
	double x2 = x0 + delta;
	double y1 = pf(x1);
	double y2 = pf(x2);
	return (y2 - y1) / (x2 - x1);
}