自然数前n项的平方和
https://blog.arg.pub/2022/02/18/math/%E5%89%8Dn%E9%A1%B9%E5%B9%B3%E6%96%B9%E5%92%8C%E6%80%8E%E4%B9%88%E6%B1%82/#more
https://zhuanlan.zhihu.com/p/436298006
https://blog.arg.pub/2022/02/18/math/%E5%89%8Dn%E9%A1%B9%E5%B9%B3%E6%96%B9%E5%92%8C%E6%80%8E%E4%B9%88%E6%B1%82/#more
https://zhuanlan.zhihu.com/p/436298006
