友友们，这个用iostream怎么写啊？对于初学者简单一点。

初学者，一些简单的，if while for之类的
自己写的，有点烂，错的离谱
#include
using namespace std;
int main()
{
int sum=0, n, a=1;
cin >> n;
for (a = 1; a <= n; a++)
{
a = 1 / (3 * n - 2);
sum = sum + a;
}
cout << "sum=" << sum<
return 0;
}

#include <iostream>
using namespace std;

int main()
{
    int n, i;
    double sum = 1;
    cin >> n;
    for (i = 2; i <= n; i++)
    {
        sum += 1.0 / (3 * i - 2);
    }
    cout << "sum=" << sum;
    return 0;
}

#include <iostream>
using namespace std;
int main()
{
    int n, a = 1;
    double sum = 0;
    cin >> n;
    for (a = 1; a <= n; a++)
    {
        sum += (double)1 / (double)(3 * a - 2);
    }
    cout << "sum=" << sum << endl;
    return 0;
}