Paul's Online Notes
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Common Math Errors

Originally the intended audience for this was my Calculus I students as pretty much every error listed here shows up in that class with alarming frequency. After writing it however I realized that, with the exception of a few examples, the first four sections should be accessible to anyone taking a math class and many of the errors listed in the first four sections also show up in math classes at pretty much every level. So, if you haven’t had calculus yet (or never will) you should ignore the last section and the occasional calculus examples in the first four sections.

I got the idea for doing this when I ran across Eric Schechter’s list of common errors located at There is a fair amount of overlap in the errors discussed on both of our pages. Sometimes the discussion is similar and at other times it’s different. The main difference between our two pages is I stick to the level of Calculus and lower while he also discusses errors in proof techniques and some more advanced topics as well. I would encourage everyone interested in common math errors also take a look at his page.