树状数组做区间加区间求和

\[Ans=\sum_{i=1}^{x}a_i\\ =\sum_{i=1}^{x}\sum_{j=1}^{i}d_j\\ =\sum_{i=1}^{x}(x+1-i)d_i\\ =\sum_{i=1}^{x}(x+1)d_i-\sum_{i=1}^{x}i\times d_i\\ =(x+1)\sum_{i=1}^{x}d_i-\sum_{i=1}^{x}i\times d_i \]

左右两个 \(\sum\) 都各自用树状数组维护即可