private void button1_Click(object sender, EventArgs e)
{
this.Hide();
frm_DangNhap dn = new frm_DangNhap();
dn.ShowDialog();
}
private void button3_Click(object sender, EventArgs e)
{
this.Hide();
frm_DangNhap tk = new frm_DangNhap();
tk.ShowDialog();
}