public void ResetForm()
 {
     this.pnlChucNang.Controls.Clear();
     tra = new TraDia(this);
     ThemGiaiDoan(tra);
     ShowForm();
 }
        private void trảĐĩaToolStripMenuItem_Click(object sender, EventArgs e)
        {
            TraDia frmTraDia = new TraDia();

            foreach (Form frm in MdiChildren)
            {
                frm.Close();
            }
            frmTraDia.MdiParent = this;
            frmTraDia.Show();
        }