2 sentences

1. 该函数的定义域是一个闭区间。

   gāi hán​shù de dìng​yì​yù shì yī gè bì​qū​jiān.

   The domain of this function is a closed interval.

   Source: Dong Chinese Dictionary

2. 这个函数的定义域是所有实数。

   zhè​ge hán​shù de dìng​yì​yù shì suǒ​yǒu shí​shù.

   The domain of this function is all real numbers.

   Source: Dong Chinese Dictionary