从1开始不断进行1.0/(n*n)的求和。如果与3.14159265相差10的-5次方则结束
int main()
{
double pi = 3.14159265;
double r=0;
double i=1;
double p = 0;
while(fabs(p-pi) >= 1e-5)
{
r += 1.0/(i*i);
i++;
p = sqrt(6*r);
}
printf("%lf,%lf,%d",r,sqrt(6*r),(int)(i-1));
return 0;
}