2-4 求交错序列前n项和

//求交错序列前n项和 #include<stdio.h>int main(int argc,char const *argv[]){int n;scanf("%d",&n);int i;double flag=1.0,sum=0;for(i=1;i<=n;i++){sum=sum+flag*i/(2*i-1);flag=-flag;}printf("%f",sum);return 0;}