请问为什么输出是0？哪里错了？

Question

#include
#include
using namespace std;
int main()
{
double x = 0;
double sum = 0;
double a = 1;
int n;
for (n=1;fabs(x)>1e-8;n++)
{

    x = (a) / (2 * n - 1);
    sum = sum + x;
    a=-a;

}
cout << "Π/4的近似值=" << sum << endl;

}

Answer 1

虽然看不太懂 但应该不会执行for循环里面的，条件似乎不满足