2-8

/*  计算 1-1/3+1/5-1/7+...共n项之和  */
#include<stdio.h>
int main(void)
{
    int denominator,flag,i,n;
    double item,sum;


    printf("Enter n:");
    scanf("%d",&n);
    /* 执行循环前，给变量赋初值 */
    flag=1;
    denominator=1;
    sum=0;
    /* 用for语句实现循环，循环执行n次 */
    for(i=1;i<=n;i++){
        item=flag*1.0/denominator;
        sum=sum+item;
        flag=-flag;
        denominator=denominator+2;
    }
    printf("sum=%f\n",sum);


    return 0;
}