Mayang Sari. Student’s Understanding of Concepts Based on APOS Theory on the material of triangles and quadrilaterals in terms from the level of mathematical ability. CEJou [Internet]. 2025 Mar. 8 [cited 2026 Oct. 12];5(1). Available from: https://ejournal.unupasuruan.ac.id/index.php/cejou/article/view/606