using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Calculator
{
    public static class MathHelper
    {

        #region Combinations
        public static double Combinations(double n, double k)
        {
            k = Math.Min(k, n - k);
            double a = 1, b = 1;

            for (int i = 1; i <= k; i++)
            {
                a *= n - i + 1;
                b *= i;
            }

            return a / b;
        }
        #endregion

        public static double Arrangements(double n, double k)
        {
            double a = 1;
            for (int i = 0; i < k; i++) a *= n - i;

            return a;
        }

        public static double Factorial(double n)
        {
            double a = 1;
            for (int i = 1; i <= n; i++) a *= i;

            return a;
        }
    }
}