Calculate the SEN of X and COSSINE of X and, calculate PI

main.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace seno
{
    static class Program
    {
        public static decimal Factorial(this int number)
        {
            return number == 0
                       ? 1
                       : Enumerable.Range(1, number).Aggregate((i, j) => i * j);
        }

        public static IEnumerable<int> PrimeNumbers(int startRange, int count)
        {
            IEnumerable<int> range = Enumerable.Range(startRange, count);
            return from n in range
                   let w = (int)Math.Sqrt(n)
                   where Enumerable.Range(2, w).All((i) => n % i > 0)
                   select n;

        }

        public static double ComputePI_linq(int num_steps)
        {
            return Enumerable.Range(0, num_steps).Aggregate(0d, (tot, next) => tot += Math.Pow(-1d, next) / (2 * next + 1) * 4);
        }

        static decimal ComputePI_Parallel(int num_steps)
        {
            decimal sum = 0.0M;
            decimal step = 1.0M / (decimal)num_steps;
            object monitor = new object();
            Parallel.ForEach(Partitioner.Create(0, num_steps), () => 0.0M,
            (range, state, local) =>
            {
                for (int i = range.Item1; i < range.Item2; i++)
                {
                    decimal x = (i + 0.5M) * step;
                    local += 4.0M / (1.0M + x * x);
                }
                return local;
            }, local => { lock (monitor) sum += local; });
            return step * sum;
        }

        static void Time<t>(Func<t> work)
        {
            var sw = Stopwatch.StartNew();
            var result = work();
            Console.WriteLine(sw.Elapsed + ": " + result);
        }

        public static decimal Cos(decimal theta)
        {
            return Enumerable.Range(1, 10).Aggregate(
                new { term = 1.0m, accum = 0.0m },
                (state, n) => new
                {
                    term = -state.term * theta * theta / (2 * n) / (2 * n - 1),
                    accum = state.accum + state.term
                },
                state => state.accum);
        }

        public static decimal Sin(decimal theta)
        {
            var nGrau = (Math.PI / 180);
            var primes = PrimeNumbers(3, 15);
            return primes.Select((item, index) =>
                                    new { i = (index % 2) * 2 - 1, o = item })
                           .Aggregate(theta, (result, b) =>
                                      result - b.i * ((decimal)Math.Pow(nGrau, (double)b.o) / Factorial(b.o)));
        }

        static void Main(string[] args)
        {
            Time(() => ComputePI_Parallel(1000000000));
            Time(() => ComputePI_linq(1000000000));


            var nGrau = (Math.PI / 180);
            var nSeno = 5;
            var theta = (decimal)(nGrau * nSeno);

            var sin = Sin(theta);
            var cos = Cos(theta);
            var tg = 1 / (cos / sin);

            Console.WriteLine("Seno: {0}", sin);
            Console.WriteLine("Coss: {0}", cos);
            Console.WriteLine("tang: {0}", tg);
        }
    }
}