middle element

例句：

1. 1. The middle element in this equation is the unknown variable.
   这个方程中的中间元素是未知变量。
2. 2. The design of the new product incorporates a middle element that enhances its functionality.
   新产品的设计中包含了一个中间元素，增强了其功能性。
3. 3.The study found that the middle element in the relationship between stress and health was not significant.
   研究发现，压力与健康之间的关系中的中间元素并不显著。