"Every time Americans debate the problem of states' rights, the relationship of federal power to state power -- which we are indeed having a roiling debate again -- and every time we debate not only race relations, but the very idea of what it means to be an American, multi-racial,greatly diverse society, we are debating the direct legacies of the Civil War."

VOA: special.2011.04.09

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

So, that's sort of the mathematically how we get to a, but we can also just look at the graph here, because every time we go one wavelength we can see that we're back in a maximum.

这是就我们得到a的数学,办法，但我们可以直接,从图上看，因为每次我们,经过一个波长，都回到一个最大值。

麻省理工公开课 - 化学原理课程节选