The Inverse Function Fallacy: On Sign Determination and Forgotten Fundamentals

Although inverse functions are introduced early in algebra, many students remain unaware that an inverse expression may legitimately involve a negative root. Instead, they default to assuming a positive root, overlooking the role of domain restrictions in determining the correct solution. This paper identifies this misconception as the "inverse function fallacy" and introduces a systematic approach, the Inverse Function Point method, that establishes sign determination to a single domain-based reference point. In a study of 69 STEM students at Rensselaer Polytechnic Institute, only 19% solved a sign-determination problem correctly on their first attempt. Those students were not re-tested. Of the remaining 56 who answered incorrectly, I was able to re-test 40 after teaching the proposed method. In this subgroup, accuracy rose to 80%. These results highlight both the fragility of assumed mathematical knowledge and the potential of simple, intuitive procedures to reinforce conceptual understanding.