C# 斐波那契數列 第n項數字/前n項的和

 static void Main(string[] args)
        {
            int a = Convert.ToInt32(Console.ReadLine());

            //求第n位數字是多少
            Console.WriteLine(F1(a));

            //求前n項的和
            Console.WriteLine(sum(a));
            Console.ReadKey();


        }

        /// <summary>
        /// 求第n位的數是幾
        /// </summary>
        /// <param name="a"></param>
        /// <returns></returns>
        private static int F1(int a)
        {
            if (a == 1 || a == 2)
                return 1;
            else
                return F1(a - 1) + F1(a - 2);
        }

        /// <summary>
        /// 求前n項的和
        /// </summary>
        /// <param name="num"></param>
        /// <returns></returns>
        private static int sum(int num)
        {
            int sum_new=0;
            for (int i = 1; i <= num; i++)
            {
                sum_new =sum_new+ F1(i);
            }
            return sum_new;
        }