#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)); // 以當前系統時間作為隨機種子
n = 0;
do {
n++;
x = rand()%10; // 生成一個0~9之間的隨機整數
printf("%3d", x);
}while(n<N);
printf("\n");
return 0;
}

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

#include<stdio.h>
#include<math.h>
int main(){
    long int s,a,n=0,t=0;
    printf("Enter a number:");
    while(scanf("%ld",&s)!=EOF){
        do{ 
            a=s%10;
            if(a%2==1){
                n++;
                t=a*pow(10,n-1)+t;
            }
            s=s/10;    
        }while(s!=0);
        printf("new number is:%ld\n\n",t);
        printf("Enter a number:");
        t=0;
        n=0;     
    }
    return 0;
}

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

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

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