F(x) = 1 - (x/(2*7)) + ...

#include<iostream>
#include<math.h>

using namespace std;

int tw = 2, svn = 7;
double x, current_x;
int cur = 1;

double abs(double a){return a > 0 ? a : -a;}

double next(int i)
{
    return (current_x * x)/(tw * i * svn * i);
}

int main()
{
   cout << "Enter 'X' : ";
   cin >> x;
   double eps;
   cout << "Enter 'eps' : ";
   cin >> eps;
   double answer = 1;
   current_x = -1;
   int i = 1;
   while (abs(next(i)) >= eps )
   {
       answer += next(i++);
       current_x *= -x;
   }
   cout << answer;
}