I use mathstackexchange for that purpose and do all my work in the little window where they want you to type up your question. I don't ask/answer questions on there. Mathjax will render all your stuff beautifully. If I want to save my work, I just take a screenshot.

This link below is a great reference that's worth keeping around:

https://math.meta.stackexchange.com/questions/5020/mathjax-b...