/* Find a root of f() by the bisection method. */

#include <stdio.h>

#define   EPS   1e-12                                             /* epsilon */

double   p(double);
double   root(double (*)(double), double, double);

int main(void)
{
   double   x;
   
   x = root(p, 0.0, 3.0);
   printf("%s%.16f\n%s%.16f\n",
      "Approximate root: ", x,
      "  Function value: ", p(x));
   return 0;
}

double p(double x)
{
   return (x * x * x * x * x  -  7.0 * x  -  3.0);
}

double root(double f(double), double a, double b)
{
   double   m = (a + b) / 2.0;                                   /* midpoint */

   if (f(m) == 0.0 || b - a < EPS)
      return m;
   else if (f(a) * f(m) < 0.0)
      return root(f, a, m);
   else
      return root(f, m, b);
}