关键部分代码：

public partial class frmStandCalculator : Form
    {
        public float number1 = 0;//存储前一个操作数;
        public bool flag = false;//标记单击了操作符没有，false为还未击操作符，ture为单击了操作符
        public char doflag = '+';//初始化操作符，单击了操作符时存储操作符号
        public bool pointflag = false;//标志是否单击了小数点;false为未击，true为已经单击;
        public float number2 = 0;//存储单击=号时TextNum的值
        public int n = 0;//记录是击了=键还是未击。
        public float M = 0;
        public string COPYM = "";

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 //开平方根：
        private void btnSqrt_Click(object sender, EventArgs e)
        {
            if (TxtNum.Text.Trim() != "")
            {
                float tempnumber = (float)Math.Sqrt(float.Parse(TxtNum.Text));//Math.Sqrt()结            //果为double型，所以要用float强制转换
                TxtNum.Text = tempnumber.ToString();
            }

            flag = true;
        }
        //百分号：
        private void btnPercent_Click(object sender, EventArgs e)
        {

        }
        //分数：
        private void btnFraction_Click(object sender, EventArgs e)
        {

        }
}