第五题

#include<stdio.h>
int main(void)
{
    int denominator,molecule,flag ,i,n;
    double item,sum;
    printf("Enter n:");
    scanf("%d",&n);
    flag=1;
    denominator=1;
    sum=0;
    molecule=1;
    for(i=1;i<=n;i++){
        item=flag*molecule*1.0/denominator;
        sum=sum+item;
        flag=-flag;
        denominator=denominator+2;
        molecule=molecule+1;
    }
    printf("sum=%f\n",sum);
    return 0;
}