sin and cosine in brackets??

Question

What does it mean to have brackets used like this?? Also on the other side, the same representation is used for $x-x'$ and $g(x)$. Where can I read more about this?

$$[\sin\theta_i, \cos\theta_i] = [(x - x'), g(x)] / \sqrt{(x - x')^2 + [g(x)]^2}$$

Answer 1

Looks like vector notation, a way to write two equations in one line. There is one equation for each of the two components of the vectors: \begin{align} \sin \theta_i &= \frac{x-x’}{\sqrt{(x-x')^2+[g(x)]^2}} \\ \cos \theta_i &= \frac{g(x)}{\sqrt{(x-x')^2+[g(x)]^2}} \\ \end{align}