Maths is a school subject considered to belong to more difficult subjects because it requires continuous and constant hard work and a lot of invested time. To make the students achieve better results, we need something to incite them to learn and that something is motivation. The findings and the answers about the motivation in the learning process is given to us by the theory developed in the framework of the social-cognitive approach, and that is the expectancy-value theory by Eccles and associates. According to the expectancy-value theory it is assumed that motivational beliefs of students significantly influence the quality of the educational outcomes, especially the belief about competence, as well as the subjective value that the student adds to the studied content. The goal of this thesis was to establish the role of motivational beliefs while studying concepts of maths, especially while learning about the concepts of a prism. We wanted to test the students' notion of a prism as a geometrical body, their ability to recognise it and its elements and their ability to describe it, i.e. to define it. We also wanted to test the motivational beliefs behind the study of the teaching material related to the concept of a prism and of the study of maths in general as well as to establish the connection between the motivational beliefs and the achieved conceptual change in the understanding of the concept of a prism. The results showed that the students achieved better results in the post-test than in the preliminary test, which confirmed our expectations that the students would have better results after the teaching material was covered. However, despite the noticeable progress, the results on the post-test indicated that there was still room for the enhancement of the quantity of the material learnt. Also, the results showed that the students who considered themselves to be more self-efficient and who added more value to the maths in general, as well as to the prisms, achieved better results in the post-test, but only in the task whose answer demanded a higher level of the students' cognitive involvement.