codeup_100000568_G

解答：

#include <stdio.h>
#include <math.h>
int main() {
	double down = 1;    //分母从1开始 
	double result = 0;
	double flag = 1;
	for(; fabs(1/down) > pow(10,-6); down+=2) {    //分母累加2 
		result += flag*(1/down);                   //分子始终为1 
		//printf("(%f)*(1/%f) = \n", flag, down);
		//printf("result = %10.8f\n", 4*result);
		flag *= -1;
	}
	printf("PI=%10.8f\n", 4*result);
	return 0; 
}