Series

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

/* This assumes that
  * a(1) = 1
  * a(2) = -4 / 6
  * a(3) = 7 / 11
  * and so on.
  * Modify computeTerm() if the second term is wrong.
  */
double computeTerm(int n)
{
    double sign = (n % 2) ? -1.0 : 1.0;
    double numerator = 2.0 - 3.0 * n;
    double denominator = 5.0 * n - 4.0;
    return sign * numerator / denominator;
}

double computeSeries(int n)
{
    double sum = 0;
    int i;
    for(i = 1; i <= n; i++) {
        sum += computeTerm(n);
    }
    return sum;
}

int main()
{
    int i;
    for(i = 1; i <= 100; i++) {
        printf("%d %.4f\n", i, computeSeries(i));
    }
}