C#-Code

using System;
using System.Collections.Generic;

namespace ConsoleFibonacci89
{
    class Program
    {
        const decimal real89 = 1m / 89m;
        static void Main(string[] args)
        {
            decimal added89 = Calc89(100);
            Console.WriteLine($"                      1 / 89 = {real89}");
            Console.WriteLine($"Fibonacciglieder aufsummiert = {added89}");
            Console.ReadLine();
        }

        static decimal Calc89(int n)
        {
            var fibonacci = GetFibonacciNumbers(n);
            decimal i = 1;
            decimal added89 = 0;
            foreach (ulong u in fibonacci)
            {
                decimal add = u;
                for (int j = 0; j < i; j++)
                {
                    add /= 10m;
                }
                i++;
                added89 += add;
            }
            return added89;
        }

        static IEnumerable<ulong> GetFibonacciNumbers(int n)
        {
            yield return 0;
            yield return 1;
            yield return 1;
            ulong last = 1;
            ulong current = 1;

            for (int i = 0; i < n - 3; i++)
            {
                ulong next = last + current;
                last = current;
                current = next;
                yield return next;
            }
        }
    }
}

C#-Code

using static System.Console;
using System.Numerics;

namespace aufgabe_204_fibo_89   {
    class Program   {
        static void Main()        {
            int exponent = 100;
            BigInteger erg = 0;            

            for (var i = 0; i < exponent; i++)
                erg += i.bi_fibo() * BigInteger.Pow(10, exponent - i);

            WriteLine("   " + erg); 
            WriteLine(1.0/89); //16 Nachkommastellen
            WriteLine("   " + BigInteger.Parse("100000000000000000000000000000000") / 89);  //32 Nachkommastellen
                                                                                            //ohne erste null
            ReadKey(false);
        }
    }

    public static class MyExtensions    {

        public static BigInteger bi_fibo(this int i)        {            
            BigInteger f0 = 0, f1 = 1;
            while(i-- > 0)            {
                BigInteger f = f1 + f0;
                f0 = f1;
                f1 = f;
            }
            return f0;
        }
    }
}

C#-Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using System.Text;
using System.Threading.Tasks;

namespace fibo81v2
{
    class Program
    {
        static void Main(string[] args)
        {
            BigInteger fibsum = 0;
            BigInteger fib = 0;
            BigInteger e = 0;
            BigInteger y = 1;
            int goal = 0;
            int counter = 50;  //die letzen Ziffern stimmen nicht weswegen ich die Zahl auf 50 stellen berchne (dadurch sind die ersten 32 korrekt)
            while (counter != goal)
            {
                fib = e + y;
                y = e;
                e = fib;
                counter--;
                for(int i = counter;i >1;i-- )
                {
                    fib *= 10;
                }
                Console.WriteLine(fib);
                fibsum += fib;
            }
            Console.WriteLine(fibsum);
            Console.Read();
        }
    }
}