#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>
int main(){
    int n,i,z,q1,m=0;
    for(n=100;n<=200;n++){
        
            for(i=2;i<n;i++){
                z=n%i; 
                if(z==0){
                q1=0;
                break;            
                }
                }
            if(z!=0){
                q1=1;
            }
            if(q1!=0){
            m++;
            printf(" %d",n);
                if(m%5==0)printf("\n");
            }
        }
    printf("\n");    
    printf("100~200之間共有%d個素數",m);
    
    return 0;
} 

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

#include<stdio.h>
#include <math.h>
int main(){
    double s;
    int n,x,y,a,b;
    printf("Enter n(1~10):");
    while(scanf("%d",&n)!=EOF){
        s=0;
        b=1;
        y=-1;
        for(x=1;x<=n;x++){
                for(a=2;a<=x;++a){
                    b=b*a;
                }
            y=pow(-1,x-1);
            s=s+(double)1/b*y;
            b=1;
        }
        printf("n=%d,s=%.6f\n",n,s);
        printf("\n");
        printf("Enter n(1~10):");
    }
    
    return 0;
}

#include<stdio.h> 
#include<stdlib.h>
#include<time.h>
int main(){
    int date,a,n=3;
    srand(time(0));
    date=rand()%31;
    printf("猜猜2020年12月哪一天會是你的luck day\n");
    printf("\n");
    printf("開始嘍，你有三次機會，猜吧（1~31）： ");
    while(n--){
        scanf("%d",&a);
        if(a<date){
            printf("\n");
            printf("你猜的日期早了，luck day還沒到呢\n");
             printf("再猜（1~31）："); 
        }
        else if(a>date){
            printf("\n");
            printf("你猜的日期晚了，luck day悄悄溜到前面啦\n");
             printf("再猜（1~31）："); 
        }
        while(a==date){
            printf("\n");
            printf("年輕人不講武德，猜中了，耗子尾汁\n",date); 
            return 0;
        }    
    }
    if(n==-1){
        printf("次數用完啦，偷偷告訴你：12月，你的lucky day是%d號\n",date);
    }
    
    return 0;
}