//dangnhap
 public static string KiemTraDangNhap(string tk, string mk)
 {
     if (tk == "" || mk == "")
     {
         string a = "";
         return(a);
     }
     return(DL_DangNhap.DangNhap(tk, mk));
 }
 //kiem tra doi mat khau
 public static bool DoiMatKhau(string tk, string mk)
 {
     return(DL_DangNhap.MatKhauMoi(tk, mk));
 }