private void bt_thoat_Click(object sender, EventArgs e)//thoat
{
Form main = new GiaoDienChinh();
this.Hide();
main.ShowDialog();
this.Close();
}
private void bt_thoat_Click(object sender, EventArgs e)//quay lai
{
Form dd = new GiaoDienChinh();
this.Hide();
dd.ShowDialog();
this.Close();
}