4-1

#include<stdio.h>
#include<math.h>
int main(void)
{
    int denominator,flag;
    double item,pi;
    flag=1;
    denominator=1;
    item=1.0;
    pi=0;
    while(fabs(item)>=0.001){
        item=flag*1.0/denominator;
        pi=pi+item;
        flag=-flag;
        denominator=denominator+2;
    }
    pi=pi*4;
    printf("pi=%.4f\n",pi);
    return 0;
}