I'm not objecting to CaptainBlack's proof as much as I have a question about how to get around a problem with it. Given an angle x define y:
in general because of the domain restriction we place on the atn function to make it bijective.
I can easily see how restricting the domain of the tan function would fix this, but then we aren't really using the tan function. The only way I can think of to get around THIS is to extend the atn function so that it's no longer 1:1. But then it isn't really the inverse of the tan function any longer.
I'm kinda going in circles here...
Just wondering if it wouldn't be better to prove that atn is an odd function by using something more direct.