private void comboBoxEx1_SelectedIndexChanged(object sender, EventArgs e)
{
txtthongkeNgaySinh.Text = tkBLL.Thongke_theoNgaysinh(comboBoxTK.SelectedItem.ToString()).ToString();
tkBLL.Hienthi_TKtheoNgaySinh(lvthongkeloaikh, comboBoxTK.SelectedItem.ToString());
}