Again, for the math phobics in the room, don't panic, this is just writing down in words what we've already seen a couple of times already today, well today and last time.

在座的有数学恐惧症的同学不必惊慌,这仅仅是用数学表达式来表示,我们今天学到的一些知识点罢了,其实是这一讲和上一讲的知识点

耶鲁公开课 - 博弈论课程节选

You can now collect the whole expression as something something i plus something something j.

你们现在可以推导出完整的表达式,用 i 和 j 相关的式子来表示

耶鲁公开课 - 基础物理课程节选

They come from writing i prime and j prime in terms of i and j, then sticking it in an expression and identifying the new representation.

写出用 i 和 j 来表示 i' 和 j' 的式子,再代入分量的表达式,就能得到新的式子

耶鲁公开课 - 基础物理课程节选