#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");
system("pause") ;
return 0;
}

#include<stdio.h>
#include<stdlib.h>
int main()
{
    int i,j,n,line=1,LINE=4
;
        for(i=100;i<=200;i++)
    {
        
 
for(j=2;j<i;j++)
        {
            if(i%j==0)
                break;
        }
        if(j>=i)
        {
         if(line<=LINE)
        { 
          printf("  %d",i);
          n=n+1;
          line++;
        }   
        else
        {
        printf("  %d\n",i);
        n=n+1;
        line=1;
    }
    }
}
printf("\n101~200之間共擁有%d個素數\n",n);
system("pause");
return 0;
}

#include<stdio.h>
#include<math.h>
int main(){
    long int a,b,n=0,t=0;
    printf("Enter a number:");
    while(scanf("%ld",&a)!=EOF){
        do{ 
            b=a%10;
            if(b%2==1){
                n++;
                t=b*pow(10,n-1)+t;//從末位數字向高位遞進，將高位奇數加到數字之前
            }
            a=a/10;    
        }while(a!=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 d,guess,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",&guess);
        if(guess<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(guess>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(guess==d){
            printf("\n恭喜你，猜對了。12月，你的luck day是%d號\n",guess);
            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;
}