In this research, we aim to develop a curriculum for teaching mathematical proofs at the secondary school level. In this paper, we use Sylvester’s theorem to plan teaching experiments designed to teach students how to think about uniqueness, existence, and mapping. As part of our research, we studied the lessons currently given in secondary school. The results suggest that it is difficult to determine whether students have learnt these ways of thinking and, therefore, to plan lessons that are intended to teach them. It can also be suggested, however, that students should experience such kinds of teaching. In future, we must consider incorporating these thinking lessons into existing lessons and curricula.