public static double TinhChuVi(TAMGIAC tg)
        {
            double a, b, c;

            a = XL_DIEM.TinhKhoangCach(tg.A, tg.B);
            b = XL_DIEM.TinhKhoangCach(tg.B, tg.C);
            c = XL_DIEM.TinhKhoangCach(tg.C, tg.A);

            return(a + b + c);
        }
        public static TAMGIAC NhapTamGiac()
        {
            TAMGIAC tg;

            Console.WriteLine("Nhap dinh A:");
            tg.A = XL_DIEM.NhapDiem();
            Console.WriteLine("Nhap dinh B:");
            tg.B = XL_DIEM.NhapDiem();
            Console.WriteLine("Nhap dinh C:");
            tg.C = XL_DIEM.NhapDiem();

            return(tg);
        }
        public static double TinhChuVi(DAGIAC dg)
        {
            double chuVi = 0;
            int    N     = dg.dsDinh.Length;

            for (int i = 0; i < N - 1; i++) //khong tinh dinh cuoi cuoi
            {
                chuVi = chuVi + XL_DIEM.TinhKhoangCach(dg.dsDinh[i], dg.dsDinh[i + 1]);
            }
            //Khoang cach giua dinh dau tien va dinh cuoi cung
            chuVi = chuVi + XL_DIEM.TinhKhoangCach(dg.dsDinh[0], dg.dsDinh[N - 1]);

            return(chuVi);
        }
        public static DAGIAC NhapDaGiac()
        {
            DAGIAC dg;

            Console.WriteLine("Nhap so dinh cua da giac");
            int N = int.Parse(Console.ReadLine());

            dg.dsDinh = new DIEM[N];
            for (int i = 0; i < N; i++)
            {
                Console.WriteLine($"Nhap dinh {i + 1}:");
                dg.dsDinh[i] = XL_DIEM.NhapDiem();
            }

            return(dg);
        }