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In this paper, we analyse the arguments given by teachers in training by justifying the truthfulness of statements referring to the derivability of a function at one point. Using an adaptation to Toulmin's model, we focus on the guarantee or justification given and whether support is presented. We identify the mathematical structure on which the justification is based, how it is used and how the argument is represented. Findings reveal that, to argue, mathematical results or rules used are often used without backing. Likewise, we detect invalid arguments, mainly because the necessary conditions for derivability are not considered. [ABSTRACT FROM AUTHOR]