C/C++ :: Find Values For Arctan Of X Using Taylor Series

I'm trying to write a program to find values for arctan of x by using taylor series. An initial value of x is given by the user and then it should print solutions from arctan(x) to arctan(1) in increments of x+0.1. It prints correctly but gives incorrect values after the initial x. I'm new to c and need some way to 'reset' the functions f1 and f2 for each increment of x (I think...)

Here's the code

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <iostream>
#include <fstream>
using namespace std;
int main() {
/* Define the beginning of the program and each variable. Also opens a text file to be written to */
FILE *f = fopen("arctan.txt", "w");

[Code] ....