You can go ahead and use that equation, or you could figure it out every time, because if you know the total number of nodes, and you know the angular node number, then you know how many nodes you're going to have left.

你们可以直接用这个方程，或者每次都自己算出来，因为如果你们知道了总的节点数，又知道角向节点数，就知道剩下的节点数是多少。

麻省理工公开课 - 化学原理课程节选

if you don't want to use that, you can also derive it as we did every time, it should intuitively make sense how we got there. But the exams are pretty short, so we don't want you doing that every time, so we'll save the 2 minutes and give you the equations directly, but it's still important to know how to use them.

吸收的和发射的，如果你不想用公式,你也可以每次都向我们这样推导它,很直观的就能得到结果，但考试时间很短，我们不希望你们每次都推导,所以我们会直告诉你，让你节省2分钟，但知道如何应用它十分重要。

麻省理工公开课 - 化学原理课程节选