2018-II-04

14-06-2023 app.cch 尚無留言

答案：A

\begin{array}{cl} \frac{1}{3 x + 7} - \frac{1}{3 x - 7} \\ = & \frac{(3 x - 7) - (3 x + 7)}{(3 x + 7) (3 x - 7)} \\ = & \frac{3 x - 7 - 3 x - 7}{9 x^{2} - 49} \\ = & \frac{- 14}{9 x^{2} - 49} \\ = & \frac{14}{49 - 9 x^{2}} \end{array}