private void TraSach_Click(object sender, EventArgs e)
        {
            frmIDDG DG = new frmIDDG();

            DG.FLag = "Tra";
            this.Hide();
            DG.Show();
        }
        private void BtnMuonsach_Click(object sender, EventArgs e)
        {
            frmIDDG DG = new frmIDDG();

            this.Hide();
            DG.FLag = "Muon";
            DG.Show();
        }