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I sometimes have the problem that I cannot find an existing answer in Stack Exchange. So I would really like to "improve" a question by adding keywords to make it easier to find.
Is there such a possibility? I think tags are not suitable for this...right?
Tags are exactly the thing here, assuming they are the right ones for the question. Although if the question needs an edit to fix any grammar or spelling issues, be sure to do that too, just don't change it so radically it no longer looks like the original poster's question.
Note that creating a new tag won't make a question more discoverable because it will be the only question with that tag.
In any case what you're actually trying to do is add a tag that's already a synonym of an existing tag i.e. formula is a synonym of mathematics and the question already has that tag. That's the real reason you can't add that particular tag to the question.
