private void cmbgt_SelectedIndexChanged(object sender, EventArgs e)
{
if (a == 0)
{
textBoxthongketheoGT.Text = tkBLL.Thongke_theogt(cmbgt.Text.Trim()).ToString();
tkBLL.Hienthi_TKtheogt(lvthongkeloaikh, cmbgt.Text.Trim());
}
}