P39-习题2-5

/*输入一个正整数n，计算1-（2/3)+(3/5)-(4/7)+(5/9)-...的前n项和*/
#include<stdio.h>
int main(void)
{
    int i,n,numerator,denominator,flag;
/*numerator是分子，denominnator是分母，flag是常数系数*/
    double sum,item;
/*sum是总和，item是各项的值*/

    printf("Enter n:");
    scanf("%d",&n);
    numerator=1;
    denominator=1;
    sum=0;
    flag=1;

/*循环n次求解各项并相加求和*/    
    for(i=1;i<=n;i++){
        item=flag*(numerator*1.0/denominator);
        numerator=numerator+1;
        denominator=denominator+2;
        flag=-flag;
        sum=sum+item;
    }
    printf("sum=%lf\n",sum);

    return 0;
}