approximate variance of the ratio estimate

例句：

1. 1.The temperature is approximately 20 degrees Celsius.
   温度大约是20摄氏度。
2. 2.The book is approximately 300 pages long.
   这本书大约有300页长。
3. 3. The distance between the two cities is approximately 50 miles.
   这两城市之间的距离大约是50英里。