#include <math.h>
#include <stdio.h>

int main(){
    float a,b,c,x1,x2;
    float delta,real,imag;
    
    printf("Enter a,b,c,:");
    
    while(scanf("%f%f%f",&a,&b,&c)!=EOF){
        if(a==0)
        printf("not quadratic equation.\n\n");
        else{
            delta=b*b-4
 
*a*c;
            
            if(delta>=0){
            x1=(-b+sqrt(delta))/(2*a);
            x2=(-b-sqrt(delta))/(2*a);
            printf("x1=%.2f,x2=%.2f\n\n",x1,x2);
            }
            else{
                real=-b/(2*a);
                imag=sqrt(-delta)/(2*a);
                printf("x1=%.2f+%.2fi,x2=%.2f-%.2fi\n\n
 
",real,
                imag,real,imag);
            }
        }
        printf("Enter a,b,c:");
    }
    return 0;
            
        
    }

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define N 5

int main() {
    int x,n;
    
    srand(time(0));
    n=0;
    do{
        n
 
++;
        x=rand()%10;
        printf("%3d",x);
    }while(n<N);
    
    printf("\n");
    
    return 0;
}
        

#include <stdio.h>
#include<stdlib.h>
#include<math.h>
int main()
{
    int n;
    int i,l;
    int h=0;
    for(n=101;n<=200;n++)
    {
        
        for(i=2; i<=sqrt(n); i++)
        {
            if(n%i == 0)
            break;
        } 
        
        if(i>sqrt(n))
            {
              h++;
              printf("%d ",n);
              l++;
               if(l%5==0)
              printf("\n");
            }
            
            
         
    }
      
      
        
    printf("\n101~200之間共有%d個素數",h);
    return 0;
}

#include<stdio.h>
#include<stdlib.h>
int main() {
    int a,b,n;
        printf("Enter a number:");
        while(scanf("%d",&n)!=EOF)
    {   
        int c=0,i=1;
        while(n >= 1) 
        {
            a = n%10;
            n = n/10;
            if(a%2 == 0);
            else 
            {
                c = c+a*i;
                i = i*10;
            }
        }
        printf("new number is :%d\n\n",c);
        printf("Enter a number:");
    }
        return 0;    
}

#include<stdio.h>
#include<math.h>
#include<stdlib.h> 
int main()
{
    int n, i, j;
    double s = 0, b;
    printf("Enter n(1~10):");
    while(scanf("%d", &n)!=EOF)
    {
        if(n >= 1 && n <= 10)
        {    
            for(i = 1; i <= n; i++)
            {
                int t = 1;
                
                for(j = 1; j <= i; j++)
                    t = t*j;
                    
                b = 1.0/t;
                
                if(i%2 == 0)
                    b = (-1)*b;
                    
                s = s+b;
            
            }
            printf("n=%d, s=%lf\n\n",n,s);
        }
        printf("Enter n(1~10):");
    }    
    
    
    return 0;
}

#include<stdio.h>
#include<stdlib.h>
#include<time.h>

int main()
{    int b,c = 1;
    srand(time(0));
    int a = rand()%30+1;
    printf("猜猜2020年12月哪一天會是你的luck day\n\n");
    printf("開始嘍，你有三次機會，猜吧（1~31）：");
    scanf("%d",&b);
    while(c <= 3)
    {
            if(c <= 3&&c!=1)
        {
            printf("\n再猜（1~31）：") ;
            scanf("%d",&b);
        }
        if(b > a)
            printf("\n你猜的日期晚了，luck day悄悄溜到前面啦\n\n");
        else 
            if(b == a)
                printf("\n答對了\n");
            else 
                printf("\n你猜的日期早了，luck day還沒到呢\n\n");
    
        c++;            
    }
    printf("\n次數用完啦。偷偷告訴你：12月，你的luck day是%d號\n",a);
    return 0;
    
}