<![CDATA[Kemampuan penalaran matematika esensial untuk berpikir logis dan sistematis, namun siswa sering kesulitan dalam materi bangun ruang sisi datar karena kurangnya pemahaman konsep. Perbedaan kemampuan matematika antar siswa mempengaruhi penalaran, dengan siswa berkemampuan rendah cenderung menghafalkan rumus. Penelitian ini bertujuan menganalisis penalaran matematika siswa dalam menyelesaikan soal bangun ruang sisi datar, ditinjau berdasarkan kemampuan matematika dengan menggunakan pendekatan deskriptif kualitatif. Data dikumpulkan melalui tes dan wawancara kepada tiga subjek (tinggi, sedang, dan rendah). Hasil menunjukkan bahwa penguasaan materi yang kuat berkorelasi positif dengan penalaran efektif, terbukti pada subjek berkemampuan tinggi yang menguasai semua indikator penalaran. Sebaliknya, subjek berkemampuan sedang dan rendah mengalami kesulitan signifikan akibat deficit pemahaman konsep dasar, yang membatasi kemampuan mereka dalam mengidentifikasi langkah, mengintegrasikan konsep, dan menyusun argument. Kontibusi penelitian ini memberikan gambaran mendalam tentang karakteristik penalaran pada berbagai tingkat kemampuan, menjadi acuan untuk pengembangan pembelajaran matematika yang adaptif, dan berorientasi pada pemahaman konseptual. Mathematical reasoning ability is essential for logical and systematic thinking, but students often have difficulty in flatsided geometric geometry material due to lack of conceptual understanding. Differences in mathematical ability between students affect reasoning, with low-ability students tending to memorize formulas. This study aims to analyze students' mathematical reasoning in solving flat-sided geometric geometry problems, reviewed based on mathematical ability using a qualitative descriptive approach. Data were collected through tests and interviews with three subjects (high, medium, and low). The results showed that strong mastery of the material was positively correlated with effective reasoning, as evidenced by high-ability subjects who mastered all reasoning indicators. In contrast, medium and lowability subjects experienced significant difficulties due to deficits in understanding basic concepts, which limited their ability to identify steps, integrate concepts, and construct arguments. The contribution of this study provides an indepth picture of the characteristics of reasoning at various levels of ability, becoming a reference for the development of adaptive mathematics learning, and oriented towards conceptual understanding]]>
