简单易懂，不用分奇偶
牛牛学数列3
http://www.nowcoder.com/questionTerminal/f65c726d081c4160a9356eabf0dc21d9
include<stdio.h>
include<math.h>
int main(){
int n ;
double i= 0.0;
double j= 0.0;
scanf("%d",&n);
for(i=0;i<n;i++)
j=j+pow(-1.0,i)/(i+1);
printf("%.3f",j);
}
