We describe the specialized knowledge of an expert Primary Education math teacher while teaching the topic of rational numbers to students of 11-12 years old. Using a category system of the subdomains of the Mathematics Teacher's Specialized Knowledge model (MTSK), a content analysis of the transcripts of 21 classes taught was performed. This allowed us to appreciate the knowledge expressed by the teacher in his practice, especially in three subdomains: mathematical content knowledge, characteristics of the learning of mathematics, and knowledge of mathematics teaching.
Mathematical Knowledge for Teaching; Mathematics Teacher Specialized Knowledge; Expert Teacher; Rational Numbers; Case Study
