Hi,

(Edited)

The full theorem is: for some integer k. In other words, x=2k\pi+a)\lor (x=2k\pi-a)" alt="\cos x=\cos a \Rightarrow \exists k\in\mathbb{Z}x=2k\pi+a)\lor (x=2k\pi-a)" />. This is much different from . Among other things, this would allow us to easily show that .

Regarding writing math formulae, etc., you can see the LaTeX tutorial within the LaTeX Help Subforum.