\[\sum_{i = 1}^{x}\sum_{j = 1}^{i}b[i]\]\[=\sum_{i = 1}^{x}(x - i + 1) * b[i]\]\[=(x + 1)\sum_{i = 1}^{x}b[i] - \sum_{i = 1}^{x}i * b[i]\]
temp
原文：https://www.cnblogs.com/Tony-Double-Sky/p/9577828.html
