This study was conducted to evaluate the activities designed to enable pupils to discover triangular inequality. The article reports the behaviors occurred while the pupils were doing these activities. It was a qualitative study utilizing participant observation, probe type questions, and worksheets. The participants were 8th grade pupils in a primary school. According to the data obtained in this research, some behaviors related to pupils’ conceptions of triangle, caused problems on the way of constructing triangular inequality. These behaviors are explained and some implications for teaching are discussed in the paper.

Keywords: triangular inequality, learning geometry, primary mathematics education