U.S. students' poor performance in the domain of geometric transformations is well documented, as are their diffi culties applying transformations to similarity tasks. At the same time, a transformations-based approach to similarity underlies the Common Core State Standards for middle and high school geometry. We argue that engaging teachers in this topic represents an urgent but largely unmet need. The article considers what a transformations-based approach to similarity looks like by contrasting it with a traditional, static approach and by providing classroom examples of students using these different methods. In addition, we highlight existing professional development opportunities for teachers in this area.
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