//ktra dang nhap neu co tra ve ma vai tro
public static string DangNhap(string tk, string mk)
{
string sql = string.Format("Select MaVaiTro from TAIKHOAN Where TenTaiKhoan = '{0}' and MatKhau = '{1}'", tk, mk);
return(DL_LayDuLieu.ScalarKieuString(sql));
}