find av for lower traingle diameter

#include<iostream>
using namespace std;
int main()
{
    int i,j,a[3][3];
    //input array
    for(i=0;i<=2;i++)
    {
        for(j=0;j<=2;j++)
        {
            cout<<"["<<i<<"]["<<j<<"]";
            cin>>a[i][j];
        }
    }
    //output array
    for(i=0;i<=2;i++)
    {
        for(j=0;j<=2;j++)
        {
            cout<<a[i][j]<<"\t";
        }
        cout<<endl<<endl;
    }
    //procces find av for lower traingle diameter
    int n=0,x=0,sum=0;
    for(i=0;i<=2;i++)
    {
        for(j=0;j<=n;j++)
        {
            sum+=a[i][j];
            x++;
        }
        n++;
    }
    float av=sum/x;
    cout<<"Av = "<<av;
}