Найти сумму членов арифметической прогрессии

#include <iostream.h>
#include <conio.h>
#include <math.h>
int main()
{
clrscr();
int a,d,n,sum;
cout<<"vvedite a"<<endl;
cin>>a;
cout<<"vvedite d"<<endl;
cin>>d;
cout<<"vvedite n"<<endl;
cin>>n;
sum=(a+n*d)*(a+n*d-d);
cout<<sum;
getch();
}

#include <iostream>
using namespace std;
 
int main(){
    int n = 0;
    float a = 0.0;
    float d = 0.0;
    float sum = 0.0;
 
    cin >> a >> d >> n;
    cout << endl;
    for (int i = 1; i <= n; i++){
        sum += a + (i - 1) * d;
        cout << sum << endl;
    }
    
    system("pause");
}

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

#include <iostream.h>

#include <iostream>

clrscr();

#include <iostream>
#include <cmath>
#include <conio.h>
 
using namespace std;
int main()
{
 
int a,d,n,sum;
cout<<"vvedite a"<<endl;
cin>>a;
cout<<"vvedite d"<<endl;
cin>>d;
cout<<"vvedite n"<<endl;
cin>>n;
sum=n*(2*a+(n-1)*d)/2;
cout<<sum;
getch();
return 0;
}