private void Sum_Click(object sender, EventArgs e)
{
  int x = System.Convert.ToInt32(textBox1.Text, 10);
  int sign = 1, i;
  float s = 0, pow, sum1 = 0;
  pow = 1;
  for (i = 1; i <= 10; i++)
  {
     pow *= x;
     sum1 += i * pow;
     s += (float)1 / sum1 * sign;
     sign = -sign;
   }
   label2.Text = "Sum is   " + s.ToString();
}
