![undefined](undefined)計算 n 個數的和
private void button1_Click(object sender, EventArgs e)
{
int n, sum, i;
n = int.Parse(textBox1.Text);
for (sum = 0, i = 1; i <= n; i = i + 1)
{
sum = sum + i;
}
label1.Text = sum.ToString();
}