using System; namespace studywork2{    public class work    {        public static void sumjiecheng()        {            long n = 1, t = 1, s = 0;            for (; n <= 20; n++)            {                t *= n;                s += t;             }             Console.WriteLine("1到20的阶乘之和为:");            Console.WriteLine(s);        }         public static void ifxushu(double n)        {            double t = 0, i = 2;            for (; i < n; i++)            {                t = n % i;                if (t == 0)                {                    Console.WriteLine("输入的数不是质数.");                    break;                }            }            if (i == n) Console.WriteLine("输入的数是质数.");        }         public static void quickpi()        {            int N = 100;             int M = 500;             int B = 100;             int L = 4;             // Machin公式             int[] s = new int[M / L];             int[] r1 = new int[N];            int[] r2 = new int[N];            int[] r3 = new int[N];            int[] r4 = new int[N];            int[] d1 = new int[N];            int[] d3 = new int[N];             int i, k, t, d2, d4, mp = M / L / 20;             double Start, Finish;             r1[0] = 1;             r1[1] = 3;             r3[0] = 4;             Start = Environment.TickCount;             for (k = 0; k < M / L; ++k)            {                 t = r1[0] * B;                 d1[0] = t / 0x5;                 r1[0] = t % 0x5;                 t = r3[0] * B;                 d3[0] = t / 0xEF;                 r3[0] = t % 0xEF;                 s[k] = d1[0] - d3[0];                 int tag = 0;                 for (i = 1; i < N; ++i)                {                     t = r1[i] * B + d1[i - 1];                     d1[i] = t / 0x19;                     r1[i] = t % 0x19;                     t = r2[i] * B + d1[i];                     d2 = t / (2 * i + 1);                     r2[i] = t % (2 * i + 1);                     //                     t = r3[i] * B + d3[i - 1];                     d3[i] = t / 0xDF21;                     r3[i] = t % 0xDF21;                     t = r4[i] * B + d3[i];                     d4 = t / (2 * i + 1);                     r4[i] = t % (2 * i + 1);                     if (tag == 1)                    {                         s[k] += (d2 - d4);                         tag = 0;                     }                     else                    {                         s[k] += (d4 - d2);                         tag = 1;                     }                 }             }             for (i = M / L - 1; i >= 0; i--)            {                 while (s[i] >= B)                {                     s[i - 1]++;                     s[i] -= B;                 }                 while (s[i] < 0)                {                     s[i - 1]--;                     s[i] += B;                 }             }            Finish = Environment.TickCount;             Console.WriteLine("派的近似值为:");            Console.Write("3.");            for (i = 0; i < 28; ++i)                Console.Write(s[i]);            Console.WriteLine(" ");            Console.WriteLine("运行时间(ms):{0}", Finish - Start);        }           public static void Main()        {            double n;            Console.WriteLine("......第一题......");            sumjiecheng();            Console.WriteLine("......第二题......");            Console.WriteLine("请输入要判断的数:");            n = Convert.ToInt32(Console.ReadLine());            ifxushu(n);            Console.WriteLine("......第三题......");            quickpi();            String str = Console.ReadLine();        }    }}  

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