نبذة مختصرة : International audience ; As a semiotic representation, the numerical line is often used in school as a tool. However, it is also considered an epistemological obstacle, which can be problematic if one is not trained to use it. In this paper, we investigate the difficulties encountered by primary and secondary pre-service teachers in representing the number line of decimal numbers when numbers are given. Using a methodology with qualitative analyses, we analysed the written productions of 103 pre-service teachers when constructing these number lines. This allowed us to identify five categories of difficulties: error in the decimal writing of 1/2, just ordering the numbers, no clearly defined unit, 1/2 in the middle of the segment, and difficulties with 1/100 and 5/100.
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