//1、編寫程序

#include<stdio.h>
main()
{
int n,i,j,sign=1;
float fac,sum;
printf("please input value of n:");
scanf("%d",&n);
sum=0.0;
for(i=1;i<=2*n-1;i=i+2)
{
fac=1.0;
for(j=1;j<=i;j++)
fac*=j; /*計算第n項*/

sum+=sign*fac; /*求前n項的和*/
sign=-sign;
}
printf("sum=%.0f\n",sum);
}

2、七個分數

#include<stdio.h>
void main()
{
?float score[7];
?float min,max,sum,avg;
?int i;
?for(i=0,sum=0;i<7;i++)
?{
??printf("請輸入第 %d 個分數: ",(i+1));
??scanf("%f",&score[i]);
??sum+=score[i];
?}
?for(i=0;i<7;i++)
?{
??if(0==i)
??{
???min=score[i];
???max=score[i];
??}
??else
??{
???if(score[i]>max)
????max=score[i];
????if(score[i]<min)
?????min=score[i];
??}
?}
?sum-=(max+min);
?avg=sum/5;
?printf("出掉最高分和最低分該學生的平均分為 %.1f\n",avg);
}

3、鍵盤輸入一個整數

#include<stdio.h>
#include<math.h>
?int shu(long unsigned num) //判斷一個給定的數num是不是素數
?//是返回1,否則返回0
?{
??int t,i;
??t=sqrt(num)+1;
??for(i=2;i<t;i++)
??{
???if(num%i==0)
????break;
??}
??if(i==t)return 1;
???return 0;
?}

int main()
{
?int n;
?printf("請輸入任意一個正整數:");
?scanf("%d",&n);
?if (shu(n))
??printf("\t%d是素數!\n",n);
?else
??printf("\t%d不是素數!\n",n);
??return 1;
}

開玩笑，三個題并一個問題里，太雞賊了