public static void ChuongTrinh_GiaiPhuongTrinhBacHai()
        {
            double a = HTConnsole.NhapSoThuc("a=");
            double b = HTConnsole.NhapSoThuc("b=");
            double c = HTConnsole.NhapSoThuc("c=");

            if (a == 0)
            {
                Console.WriteLine("a khong the la 0");
                return;
            }
            double[] nghiem = HTMath.PhuongTrinhBac2(a, b, c);
            if (nghiem.Length == 2)
            {
                Console.WriteLine("Phuong trinh co 2 nghiem la x1 = {0}, x2 = {1}", nghiem[0], nghiem[1]);
            }
            else if (nghiem.Length == 1)
            {
                Console.WriteLine("Phuong trinh co 1 nghiem la x = {0}", nghiem[0]);
            }
            else if (nghiem.Length == 0)
            {
                Console.WriteLine("Phuong trinh vo nghiem");
            }
        }