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)>=0.0001){
        item=flag*1.0/denominator;
        pi=pi+item;
        flag=-flag;
        denominator=denominator+2;
   }
   pi=pi*4;
   printf("pi=%.4f\n",pi);

   return 0;
}