Вычислить произведение ряда

#include <iostream>
#include <cstdlib>
#include <cmath>
using namespace std;
 
int main() {
    double a, rez;
    int n, i, sum;
    cout << "Enter a" << endl;
    cin >> a;
    cout << "Enter n" << endl;
    cin >> n;
 
    rez = a * (pow((a + 1), 2));
 
    for (i = 1; i <= n; i++) {
        sum = rez*(pow((a + n - 1), n));
    }
 
    cout << "Rez =" << sum << endl;
    system("pause");
    return 0;
}

double a=5, mul=1;
int n=8;
for (int i = 1; i <= n; i++) mul*=pow(a+i-1,i);
cout << "Rez =" << mul << endl;

#include <iostream>
#include <cmath>
using namespace std;
double Func (double arg1,int arg2){
    if (arg2==0) return 1;
    return (pow((arg1+arg2-1),arg2)*Func(arg1,arg2-1));
}
int main() {
double a;
int n;
    cout << "Enter a" << endl;
    cin >> a;
    cout << "Enter n" << endl;
    cin >> n;
    printf ("%0.2f\n", Func(a,n));
    system("pause");
    return 0;
}

#include <iostream>
#include <cmath>
#include <cstdint>
using namespace std;
 
int main()
{
    int a, n;
    uint64_t r = 1;
    cout << "Enter a" << endl;
    cin >> a;
    cout << "Enter n" << endl;
    cin >> n;
 
    for (int i = 1; i <= n; i++)
        r *= static_cast<uint64_t>(pow(static_cast<long double>(a + i - 1), i));
 
    cout << "Rez = " << r << endl;
}