课后题3，4

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

namespace 练习2
{
    class Program
    {
        static void Main(string[] args)
        {
            //输入3个数，将这三个数从大到小排列打印出来；
            Console.Write("请输入第一个数:");
            int a =Convert .ToInt32 (Console.ReadLine ());
            Console.Write("请输入第二个数:");
            int b = Convert.ToInt32(Console .ReadLine ());
            Console.Write("请输入第三个数:");
            int c = Convert.ToInt32(Console .ReadLine ());

            if (a >= b && a >= c)
            {
                if (b >= c)
                {
                    Console.WriteLine(a);
                    Console.WriteLine(b);
                    Console.WriteLine(c);
                }
                else
                {
                    Console.WriteLine(a);
                    Console.WriteLine(c);
                    Console.WriteLine(b);
                }
            }
            if (b >= a && b >= c)
            { 
            if (a >=c)
            {
                Console.WriteLine(b);
                Console.WriteLine(a);
                Console.WriteLine(c);
            }
                else 
            {
                Console.WriteLine(b);
                Console.WriteLine(c);   
                Console.WriteLine(a);
            }
            }
            if (c >= b && c >= a)
            {
                if (b >= a)
                {
                    Console.WriteLine(c);
                    Console.WriteLine(b);
                    Console.WriteLine(a);
                }
                else
                {
                    Console.WriteLine(c);
                    Console.WriteLine(a);
                    Console.WriteLine(b);
                }
            }

            Console.ReadLine();
        }
    }

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

namespace 课后题2
{
    class Program
    {
        static void Main(string[] args)
        {

            //打印100以内的全部质数，并求和

            int count = 0;
            int sum = 0;

            for (int i = 1; i <= 100; i++)
            {
                count = 0;

                //判断当前数是否是质数
                //质数就是只能被1和自身整除的数
                for (int j = 1; j <= i; j++)
                {
                    if (i % j == 0)
                    {
                        count++;
                    }
                }

                if (count == 2)
                {
                    Console.Write(i + ",");
                    sum += i;
                }
            }

            Console.ReadLine();
        }
    }
}