Double x = Convert.ToDouble(vvod_x.Text);
 Double n = Convert.ToDouble(vvod_n.Text);
 for (int j = 0; j < n; j++);
 Double s = ((2 * j + 5) * Math.Pow(x, 2)) / (2 * Math.Abs(j));

for (int j = 0; j < n; j++);

for (int j = 0; j < n; j++)

Double x = Convert.ToDouble(vvod_x.Text);
 Double n = Convert.ToDouble(vvod_n.Text);
 for (int j = 0; j < n; j++);
 Double s = ((2 * j + 5) * Math.Pow(x, 2)) / (2 * Math.Abs(j));

Double x = Convert.ToDouble(vvod_x.Text);
 Double n = Convert.ToDouble(vvod_n.Text);
 Double s;
 for (int j = 0; j < n; j++)
 {
    s = ((2 * j + 5) * Math.Pow(x, 2)) / (2 * Math.Abs(j));
 }