Check

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

using namespace std;
int main()
{
    setlocale(LC_ALL, "RUS");
    double x, b, h, a, y, i, s0, s1, e, p, s, z, pi, j;
    pi = acos(-1.0);
    e = 0.0001;
    b = pi;
    h = pi / 25;
    a = pi / 5;
    j = (-1) * cos(pi * 0.2);
    cout << "j=" << j;
    //z = 0;
//    cout << a;
    for (x = a; x <= b; x = x + h)
    {
        s = 0;
        i = 1;
        //z++;
        //cout << "итерация for " << z << "\n";
        s0 = (pow(-1, i) * cos(i * x)) / (i * i);
        do
        {
            //cout << "итерация while " << i << "\n";
            s1 = s0;
            s = s + s1;
            i++;
            s0 = (pow(-1, i) * cos(i * x)) / (i * i); //cout << "s0=" << s0 << "\n";
            //p = fabs(s0 - y);
            //cout << "|s1-s0|=" << fabs(s1 - s0)<<"\n";
            //cout << "|s0-y|=" << p << "\n";
            //cout << "y=" << y << "\n";
        }
        while (fabs(s0-s1) >= e);

        y = ((x * x) - ((pi * pi) / 3)*0.25);

        //cout << "y=" << y << "\n";
        //cout << "|s-y|=" << fabs(s - y) << "\n";

        cout << "  " << endl;
        cout << "sum=" << s << " " << y;
    }
    return 0;
}