Mathematical Knowledge for Teaching: an experience with in-service teachers with multiplication of decimals

Teachers' professional knowledge may directly influence the learning opportunities provided for their students. This text analyses one of the components of professional knowledge, mathematical knowledge for teaching (BALL; THAMES; PHELPS, 2008), through a situation where, in a collaborative group - in a Continuing Education Program - the theoretical foundations of the multiplication of two decimal numbers are discussed. In this discussion, the lack of knowledge of the teachers involved becomes evident when they have to explain it to their students in a way that they can understand and that enables them to use the algorithms comprehensively, for example, to multiply two decimal numbers