https://bearsimple.typecho.ru/index.php/archives/22/ $\sum_{i=0}^n i^2 = \frac{(n^2 + n)(2n+1)}{6}$$\sum_{i=0}^n i^2 = \frac{(n^2 + n)(2n+1)}{6}$$\sum... https://bearsimple.typecho.ru/index.php/archives/22/#comment-25 2024-07-14T15:07:52+08:00 xxxxxxx