Pi近似值为什么不精确？

#include 
#include 

using namespace std;

int main()
{
  long int i=0;
   double sum=0,term,z,pi;
   float x=10.0,y=(-12.0);
   z = pow(x,y);
   do {i+=1;
   term=1.0/(i*i);
   sum+=1.0/(i*i);
   }
   while (term<=z);
   pi=sqrt(sum*6);
    cout<<"近似值为："<"Hello world!" << endl;
    return 0;
}

int main()
{
    long int i = 0;
    double sum = 0, term, z, pi;
    // float x = 10.0, y = (-12.0);
    // z = pow(x, y);
    do
    {
        i += 1;
        term = 1.0 / (i * i);
        sum += term; // 1.0 / (i * i);
    } while (term > 1e-12);

    pi = sqrt(sum * 6);
    cout << "近似值为：" << pi << endl;

    return 0;
}