The question means the square of i* (take i in n) i accumulate from 0 to n
Derivation formulas, combinatorics.
Original code:
#include<bits/stdc++.h>
#define ll long long
using namespace std;
ll pow_mod(ll a,ll n,ll m)
{
if(n==0)
return 1;
ll x=pow_mod(a,n/2,m);
ll ans=x*x%m;
if(n%2==1)
years=years*y%m;
return ans;
}
intmain()
{
ll n,d=1000000007;
ll sum=0;
int t;
scanf("%d",&t);
while(t--)
{
scanf("%lld",&n);
sum=pow_mod(2,n-1,d);
sum=(((n%d)*((n+1)%d))/2%d*(sum%d)%d);
printf("%lld\n",sum);
}
return 0;
}