Pages and pages of them, so no one expects you to know all those identities, but there are a few popular ones we will use.

好几页的内容,我不要求你们都能记下来,但是我们会用到一些著名的恒等式

耶鲁公开课 - 基础物理课程节选

All right. And of course, there's trigonometric identities you know from high school.

好的,当然了,你们高中就学过了三角恒等式

耶鲁公开课 - 基础物理课程节选

But since sine square plus cosine square is 1, you'll find this vector has a fixed length R.

但是由于正弦和余弦的平方和为1,你会发现这个矢量模长恒等于 R

耶鲁公开课 - 基础物理课程节选