#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+%.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<stdlib.h>
#include<math.h>
int main(){
    int i,m,n=101
 
;
    scanf("%d",&n);
    while(n>=101&&n<=200)
    {m=sqrt(n);
    for(i=2;i<=m;i++)
        if(n%i==0)break;
    if(i>m&&n>=101)
    printf("%d\n",n);
    n++;
}   
     printf("101~200之間共有21個素數\n");
     return 0;
} 

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

#include<stdio.h>
 int main(){
     int n,i,c,f;
     float s,b;
     printf("Enter n(1~10):");
    while(scanf("%d",&n)!=EOF){
        c=1,f=-1,s=0;
    for(i=1;i<=n;i++){
        f=f*(-1);
        c=c*i;
        b=1.0/c*f;
        s=s+b;
    }
    printf("n=%d,s=%f\n",n,s);
    printf("Enter n(1~10):\n");
}
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(){
int n,b=1;
srand(time(0));
int a = rand() % 30 + 1;
printf("猜猜2020年12月哪一天會是你的luck day\n開始嘍，你有三次機會，猜吧(1~31):\n");
scanf("%d",&n);
while(b<=3){
if(b<=3&&b!=1)
{
printf("再猜(1~31):\n");
scanf("%d",&n);
}
if(n>a)
printf("你猜的日期晚了，luck day悄悄溜到前面啦！\n");
else
if(n==a)
printf("猜對了\n");
else
printf("你猜的日期早了，luck day 還沒到呢！\n");
b++;
}

return 0;
}