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; 
			   }

// 生成N個0~9之間的隨機整數，並列印輸出
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define N 5

int main() {

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

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

#include <stdio.h>
int main(){
    
    int n,m;

    printf("Enter n(1~10): ");
    while(scanf("%d",&n) != EOF){
        float a=1,x=1,s=0,i=1;
        m=n;
        while(n--){
            a=a*i;
            i++;
            s=s+x/a;
            x*=-1;
        }
        printf("n = %d, s = %f\n\n",m,s);
        printf("Enter n(1~10): ");
    }
    
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(){
    
    int d,x,n=3;
    srand(time(0));
    d=rand()%31+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）： ");
    while(n--){
        scanf("%d",&x);
        if(x<d){
            printf("\n你猜的日期早了，luck day還沒到呢\n");
            if(n!=0)
                printf("\n再猜(1~31): ");
        }
        else if(x>d){
            printf("\n你猜的日期晚了，luck day悄悄溜到前面啦\n");
            if(n!=0)
                printf("\n再猜(1~31): ");
        }
        else if(x==d){
            printf("\n恭喜你，猜對了。12月，你的luck day是%d號\n",x);
            break;
        }
    }
    if(n==-1){
        printf("\n次數用完啦。悄悄告訴你: 12月，你的luck day是%d號\n",d);
    }
    
    return 0;
}

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