计算两个数的和（窗体应用程序）直接在窗体中显示
laber:a,b,c textBox:输入值的地方 button:求和，求差
求和：
private void button1_Click(object sender, EventArgs e)
{
float a, b, s;
a = float.Parse(this.textBox1.Text);
b = float.Parse(this.textBox2.Text);
s = a + b;
this.textBox3.Text = s.ToString();
}
求差：
private void button2_Click(object sender, EventArgs e)
{
float a, b, s;
a = float.Parse(this.textBox1.Text);
b = float.Parse(this.textBox2.Text);
s = a - b;
this.textBox3.Text = s.ToString(); //在窗体中显示
}