binsearch-func

#define EPS 1E-9

double f(double x)
{
   //монотонная функция на отрезке [a, b], например f(x) = x^3:
   return x*x*x;
}

double binarySearch(double C, double a, double b)
{
   double low = a, high = b;
   while(abs(low-high) < EPS)
   {
      double mid = low + (high - low) / 2;
      if f(mid) < C
          low = mid;
      else
          high = mid;
   }
   return (low + high)/2;
}