I've posted a few questions in the Mathematics Stack Exchange community asking about theoretical concerns, such as "Why is $\frac{\frac{1}{x}}{\frac{1}{x^2}+1}=\frac{x}{x^2+1}$ true for all $x$?" Oftentimes on that community, such a question gets closed because it is "off topic." There is usually no referral made to a more appropriate forum/community, just closing, even if there is an answer posted already. Because I have not had much success with the Mathematics Stack Exchange community, I've decided to try this one. Is theoretical math on topic for MathOverflow, or do I need to post elsewhere? If so, where?

$\begingroup$ this is the only question of yours that has been closed on MSE, isn't it? so I'm unsure what you mean by your "not having had much success with the Mathematics Stack Exchange community". $\endgroup$ – Carlo Beenakker Feb 13 at 22:26
No, it isn't.
What kind of questions can I ask here?
MathOverflow's primary goal is for users to ask and answer mathematical questions related to current research in mathematics. These can be questions that arise when you are working on or preparing to work on a mathematical researchproblem, but they also may be questions that you come across when you are reading scholarly journals or advanced graduate level books in mathematics.
(source)
This is not researchlevel mathematics; it may be theoretical but most of mathematics is theoretical. IMHO, it should be ontopic on Mathematics Stack Exchange but it seems it has been closed because five users think you didn't provide enough context. That can mean a lot of things, but you have to ask on Mathematics Meta in order to find out.