From 1968 to 1975 the centralized Portuguese school system began with six years of mandatory education (6-11 years old). Those that wanted to pursue education should choose between secondary schools (liceus) and technical schools. From 1976, a unification of the remaining secondary school years was implemented, and the secondary education comprised a General Course – grades 7, 8 and 9 – and a Complementary Course – grade 10 and 11. The establishing of the Unified Secondary Course required curricular changes, and new mathematics programs are issued. As a consequence, three mathematics textbook collections for the General Course gradually appeared on
the market. In the period of our study the ideas of modern mathematics were being applied in Portuguese schools. This text intends to understand different ways to interpret the mathematics programs for Unified Secondary Course, by comparing first editions of these collections of textbooks. We looked at the general structure of the texts, the intended relationship with the reader and the ways in which exercises were proposed. The present text focus on the introduction of the grade 7’s topic ‘Numerical equations of the first degree in Q’ and identifies differences and similarities in the approach to this concept. Results show that two of the collections were organized to promote the active participation of students. Observing the introduction of the topic, there are similarities in the approaches of two of the textbooks that used inverse operations to solve equations whereas the other used scales. It stands out that only one textbook refers to the equivalence of equations. Regarding the exercises, two of the textbooks have an Exercise Book and a Worksheet Book, respectively, to support students’ study. The third includes a list of exercises on solving an equation as activities for the students. The study was based on a documental analysis of a descriptive and interpretative nature, with a historical perspective. Our sources were legislation, programs, and textbooks.