посмотрим

#include <iostream>
using namespace std;
int main()
{
    double k,x;
    cout<<"x=";cin>>x;
    cout<<"k=";cin>>k;
    double b=x*x*x;
    double p=1.0;
    for (int n=2;n<=k;n++)
    {
        p*=1.0+b/(n*n*n-1.0);
        b*=-x*x;
    }
    cout<<"p="<<p<<endl;
    system("pause");
    return 0;
}