Posted by haifeng on 2011-06-21 18:07:54 last update 2020-01-17 07:35:41 | Answers (4) | 收藏
\[\frac{\pi}{4}=1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\cdots\]
