How do I cite an equation that has a typo in the source?
You should indicate what the typo was and that you corrected it. This will prevent your readers getting confused if/when they look up the reference, and related annoyances (e.g., the referee accusing you of making an error, your professor giving you a bad grade, or people generally just thinking you are sloppy). Noting the correction will also create a historical record of the typo, which could be useful for other researchers working on the same topic in the future.
Related discussion for a non-mathematical correction: Quoting a typo: Do I really have to do &quot;sic&quot;, or can I just fix the sentence?