#include<stdio.h>
#include<math.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<math.h>
int main()
{
    int m,i,k,h=0,leap=1;
    printf("\n");
    for(m=101;m<=200;m++)
    {
        k=sqrt(m+1);
        for(i=2;i<=k;i++)
        if(m%i==0)
        {
            leap=0;
            break;
        }
        if(leap)
        {
            printf("%4d",m);
            h++;
            if(h%5==0)
            printf("\n");
         } 
        leap=1;
    }
    printf("\n");
    printf("100~200之間共有%d個素數.\n",h);
    return 0; 
 } 
 

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

int main(){
    long int x,y=0;
    int i,n,m,s=0;
    printf("Enter a number: ");
    while(scanf("%d",&x)!=EOF){
        do{
            m=x%10;
            if(m%2!=0){
                i=pow(10,s);
                y=y+m*i;
                s=s+1;
            }
            x=x/10;
        }
        while(x/10!=0);
        printf("new number is : ");
        if(x%2!=0)
        printf("%ld",x);printf("%ld",y);
        printf("\n");printf("\n");
        
        printf("Enter a number: ");
        s=0;    y=0;
    }
    
    return 0;
}

#include<stdio.h>
#include<math.h>
#include<stdlib.h>
int main()
{
    int n,i; 
    
    printf("Enter n(1~10): ");
    while(scanf("%d",&n)!=EOF){
    double k,s=0.0,t=1.0;
    for(i=1;i<=n;i++)
    {
        if(i%2!=0)
        {
            t=abs(t*i);
        }else{
            t=-(t*i);
        }
        s+=1.0/t;
    }

    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 x,n,i;
    srand(time(0));
      n=0;
      x=rand()%31;
        printf("猜猜哪一天會是你的luck day\n\n");
        printf("開始嘍，你有三次機會，猜吧(1~31): ");
    for(i=1;i<=3;++i){
    if(i>=2){
        printf("再猜(1~31)："); 
    }
    scanf("%d",&n);
    printf("\n");
     if(n<x){
            printf("你猜的日期早了，luck day還沒到呢\n\n");
        }
         if(n==x){
            printf("你猜對了");
            break;
        }
         if(n>x){
            printf("你猜的日期晚了，luck day悄悄溜到前面啦\n\n");
        }
    if(i==3){
            printf("次數用完啦，悄悄告訴你，12月，你的luck day是%d號",x);
        }
        }
       printf("\n\n");
    return 0;
}