[en] Trigonometry, an important prerequisite for many advanced topics of school mathematics, links geometric, algebraic and graphical reasoning, but remains a difficult topic to teach and learn. The dynamic nature of many trigonometric functions is amenable to dynamic geometry software, which, in the form of GeoGebra, is the focus of this paper. However, both generally and in respect of trigonometry, research on GeoGebra's efficacy seems ambivalent. In this paper, we offer a case study of two groups of Swedish upper secondary students' solutions to the same tasks. One group was instructed to use GeoGebra and the other a protractor to investigate the sine and cosine functions in in the interval 0° ≤ v ≤ 180°. Analyses yielded qualitatively different outcomes; students using the protractor typically identified a geometrical relationship based on symmetry around the protractor's 90° line, while those using GeoGebra tended to identify only numerical relationships.