Rule for taking derivatives, if you want to do this mindless application of calculus, you can do it.

按求导法则来做,如果你想解决这个微积分的简单应用,你可以很容易做到

耶鲁公开课 - 基础物理课程节选

Light travels basically a foot in a nanosecond. Simple rule of thumb.

光行进的速度是每纳秒一英尺,很简单的经验法则了。

麻省理工公开课 - 计算机科学及编程导论课程节选