CodeForces - 476C-Dreamoon and Sums(数学思维)

C. Dreamoon and Sums

题解：设则有

题目所求
可得
所以

代码

#include<bits/stdc++.h>
typedef long long LL;
using namespace std;
const int mod = 1e9+7;
int main()
{
#ifndef ONLINE_JUDGE
    freopen("input.in","r",stdin);
#endif
    LL a,b;
    cin>>a>>b;
    LL t = ((b + 1) * a ) % mod;
    LL tmp = (a * (a - 1) / 2) % mod;
    tmp = tmp * b % mod;
    tmp = (tmp + t) % mod;
    t = (b - 1) * b / 2 % mod;
    cout<<(t * tmp) % mod<<endl;
    return 0;
}