<![CDATA[This study was motivated by the limited conceptual understanding of area and volume among prospective mathematics teachers, who often rely on procedural strategies instead of conceptual reasoning. This study aimed to develop and evaluate an interactive GeoGebra applet based on dynamic unitizing, an approach that extends GeoGebra beyond dynamic visualization by supporting conceptual construction through the manipulation of unit squares and cubes. An explanatory sequential mixed-methods design was used. The applet was developed using the ADDIE model integrated with Tessmer’s formative evaluation and implemented through one-on-one (three students), small-group (seven students), and field testing (28 students). The applet demonstrated very high validity (media: 99.48%; material: 99.31%) and high practicality, as evaluated by students (92%) and lecturers (89.93%). Students’ conceptual understanding improved significantly after using the applet (Wilcoxon Signed-Rank Test: Z = −4.541, p < 0.001), with a large effect size (r = 0.86) and moderate normalized gain (N-gain = 0.542). Qualitative findings further indicated that the applet enabled students to move beyond procedural strategies by independently exploring and constructing the area and volume concepts. These findings suggest that the applet is a valid, practical, and promising learning medium for enhancing the conceptual understanding of area and volume.]]>
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