Update 9.md

jackzhenguo · web-flow · commit efa3de2b182f · 2021-10-03T20:09:25.000+08:00
diff --git a/md/9.md b/md/9.md
@@ -17,6 +17,11 @@ In [1]: float('3')
 Out[1]: 3.0
 ```
 
+```python
+float('inf') # 正无穷大
+float('-inf') # 负无穷大
+```
+
 如果不能转化为浮点数，则会报`ValueError`:
 
 ```python
