The aim of this work is to show the results of the application of a series of exercises on validation to a group of 42 incoming college students and volunteers in careers of degree in natural sciences and engineering. The results are used to characterize the situation of this group of students in process of validation of mathematical propositions. The analysis of the kinds of answers obtained is an adaptation of the classification by Bell (1976) and also incorporates the classification done by

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... alacheff (2000) about the types of proof made by the students. The results, in general, make evidence of the ignorance of the mathematical language, which complicate the reading and in consequence hinders the process of teaching and learning in situations of validation. In addition, they also show up the confusion of the students in front of actions as dissimilar as verifying and showing.