#include <stdio.h>
#include <math.h>


double sqrt5function(double x)
{
    return x * x - 5;
}


int main()
{
    extern double secant(double (*f)(double));
    printf("%g\n", secant(sqrt5function));
    return 0;
}


double secant(double (*f)(double))
{
    double x = 1, oldx = 2;
    while (fabs((*f)(x)) > 1e-5) {
	double newx = x - (*f)(x) * (x - oldx) / ((*f)(x) - (*f)(oldx));
	oldx = x;
	x = newx;
    }
    return x;
}