求交错序列前N项和

https://pintia.cn/problem-sets/13/problems/410

 1 #include <stdio.h>
 2 int main(void)
 3 {
 4     int n;
 5     int numerator, denominator;
 6     int flag;
 7     double item;
 8     double sum;
 9 
10     numerator = 1;
11     denominator = 1;
12     flag = 1;
13     sum = 0;
14     scanf("%d", &n);
15     for (int i = 0; i < n; i++)
16     {
17         item = flag * 1.0 * numerator / denominator;
18         sum = sum + item;
19         numerator = numerator + 1;
20         denominator = denominator + 2;
21         flag = -flag;
22     }
23 
24     printf("%.3f
", sum);
25 
26     return 0;
27 }