This study aims to analyze the ability of high school students to understand concepts and mathematical reasoning
on the properties of circles. The study used a qualitative descriptive approach with a one-shot case study design,
involving 20 students in class X of SMA Negeri 5 Cirebon who were selected through purposive sampling.
Instruments in the form of post-tests are used to measure two cognitive aspects, namely concept understanding
and mathematical reasoning. The results showed that the ability to understand concepts was higher than
reasoning. As many as 80% of students were able to solve the circle area problem, and 90% were able to apply
the juring area formula correctly, with an average comprehension score of 85. However, mathematical reasoning
still varies, with an average score of 77.5; Only 75% of students can give rational reasons related to the shape
of circles, and 80% are able to relate the concept of angles to daily life, and there are even students who get a
score of 0. These findings show that conceptual mastery is not fully accompanied by reasoning skills, so it is
necessary to apply problem-based learning strategies and the use of contextual interactive media to strengthen
concept understanding while improving critical thinking skills and mathematical reasoning in a more meaningful
way.
Keywords: Circle Geometry, Concept Understanding, Mathematical Reasoning, One-Shot Case Study