private void btnMaSach_Click(object sender, EventArgs e)
{
frmSach frmS = new frmSach();
frmS.ShowDialog();
hienthimaSach();
}
private void sÁCHToolStripMenuItem_Click(object sender, EventArgs e)
{
frmSach frmSach = new frmSach();
this.Hide();
frmSach.ShowDialog();
this.Show();
}