Okay, so how do we know that?

那我们是怎么知道的呢
How do we know it's length of A

我们怎么知道是 A 的长度
times length of B times cosine of the angle?

乘以 B 的长度再乘以夹角的余弦值呢
You should follow from...

你应该按照
Is it an independent definition

它是一个独立的定义呢
or is it a consequence of this? Yep?

还是由这个式子推导出来的呢请说
Yes.

好的
He said we can derive it using the Law of Cosines

刚才那位同学说要用到余弦定理
and that's what I will do now.

这就是我接下来要做的
In other words, that definition which you may have learnt about first

换句话说也许你们先前学到的定义
is not independent of this definition;

并非一个独立定义
it's a consequence of this definition.

而是根据这个定义推导出来的