This study aims to analyze the hierarchical development of the triangle concept across elementary and secondary mathematics curricula through the theoretical lens of Van Hiele’s geometric thinking levels. The triangle is introduced in elementary school as a visual and perceptual object, reinterpreted in middle school as a structure involving relationships among sides and angles, and finally abstracted in high school into the forms of trigonometric ratios, trigonometric functions, and trigonometric inequalities. The study argues that this repeated appearance of the triangle throughout the curriculum does not represent mere content repetition, but rather reflects qualitative shifts in students’ modes of geometric reasoning. By mapping curriculum achievement standards related to triangles onto Van Hiele’s levels from visualization to formal deduction, this research demonstrates how the concept of the triangle evolves from a concrete figure to a formal mathematical structure. The findings provide theoretical support for curriculum coherence and suggest instructional strategies that promote meaningful conceptual continuity.

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