This thesis studies changes in proofs of elementary geometry theorems found in brazilian textbooks from the XVIIIth century on. The thesis is composed of two parts. The first one contains a historic survey of re-readings of Euclid´s Elements which, from the XVIth century on influenced the constitution of the school mathematics corpus, in its wide sense. The second ´part contains the actual analysis of proofs. We examine both historically important Works and books that were influential in the teaching and learning of deductive geometry in Brazil. The study of Euclid´s Elements and later works enabled us to set up the guidelines for the proofs analysis and all of more general inferences. The proofs analysis shows several types of change: the style, the method and the actual proof. It was possible to identify two basic kinds of text-books, which we call elements of geometry and mathematics books, respectively, and to establish the correlation between the two types: the deductive study of geometry, present in the elements of geometry, that adopted the Euclidean pattern of theorem - problem, changes due to the interaction between geometry, arithmetic and algebra, which characterizes the mathematics book, from their beginnings to the present. It was also possible to establish that the deductive study of geometry almost disappears in the mathematics books. The study of proofs and their changes shows the history of the contents of school mathematics, their text-books and of school mathematics in Brazil.