How does this proof of law of sines determine equal angles?

I was reading over this proof of the law of sines and they say that $\angle CAB = \angle DOB$ because of "basic geometry". I do not get it though, how can you say that the angles are equal?

• The link is not working for me. Regardless, though, it is preferable to make questions self-contained, whenever possible. Please edit your question to include relevant excerpts and diagrams. – Cameron Buie Nov 18 '13 at 20:52

So $m\angle COB = 2 m\angle CAB$, and they bisected segment $CB$ to halve the angle.
$\angle CAB$ is an inscribed angle; $\angle DOB$ is half of a central angle through the same arc, so the angles are equal.