谢谢楼上几位了啊~

while(scanf("%d",&n)==1)的意思是只要你输入数字就循环，不输入就不循环。

#include<stdio.h>
 int main(void)
 {
     int n,i,k;
     double sum1,sum2,x;
     printf("输入你想求得前n项和:");
     scanf("%d",&n);
     sum1=0;
     sum2=0;
     k=-1;
     while(n!=1)(你的上面已经输入过一次了，所以这里不应该用scanf("%d",&n)==1直接用,所以scanf("%d",&n)直接用n就行了，再者你的条件如果是输入n==1在里循环，所以应该条件改为n!=1)
     {
         for(i=1,x=1.0;i<=n;i++,x++)
         {
             k=(-1)*k;
             sum1+=1.0/x;
             sum2+=(k*1.0)/x;
         }
         printf("(1.0+1.0/2.0+1.0/3.0+1.0/4.0+...)的前%d项和=%lf.\n",n,sum1);
         printf("(1.0-1.0/2.0+1.0/3.0-1.0/4.0+...)的前%d项和=%lf.\n",n,sum2);
         printf("Enter next value:");
         scanf("%d",&n);
     }
     printf("Done!\n");
     return 0;
 }