private void frmChucNang_Load(object sender, EventArgs e)
        {
            frmThueDia f = new frmThueDia();

            f.TopLevel = false;
            pnMain.Controls.Add(f);
            f.Dock = DockStyle.Fill;
            f.Show();
        }
        private void menuThueDia_Click(object sender, EventArgs e)
        {
            pnMain.Controls.Clear();
            frmThueDia f = new frmThueDia();

            f.TopLevel = false;
            pnMain.Controls.Add(f);
            f.Dock = DockStyle.Fill;
            f.Show();
        }