//乘法
        private void button14_Click(object sender, EventArgs e)
        {
            btn1 = Convert.ToDouble(textBox1.Text);
            S1 = 4;
            textBox1.Text = "";
            
        }
  private void button17_Click(object sender, EventArgs e)
        {
             switch (S1)
            {
                case 1:
                    btn2 = btn1 + Convert.ToDouble(textBox1.Text);
                    textBox1.Text = btn1.ToString();
                    break;
                case 2:
                    btn2 = btn1 - Convert.ToDouble(textBox1.Text);
                    textBox1.Text = btn2.ToString();
                    break;
                case 3:
                    btn2 = btn1 / Convert.ToDouble(textBox1.Text);
                    textBox1.Text = btn2.ToString();
                    break;
                case 4:
                    btn2 = btn1 * Convert.ToDouble(textBox1.Text);
                    textBox1.Text = btn2.ToString();
                    break;
            }
        }