public void ThemTu()
        {
            ThemTu f = new ThemTu(this);

            this.Hide();
            f.ShowDialog();
            this.Show();
        }
        private void Them_Button_Click(object sender, EventArgs e)
        {
            ThemTu f = new ThemTu();

            this.Hide();
            f.ShowDialog();
            this.Show();
        }