• #Trigonometry
• #Mathematics
• #Trigonometric function

This study analyzed the changes in content related to plane figures in the geometry domains of Joseon-Sanhak in the late 18th century compared to previous periods. It focused on changes in explanations and calculation methods for plane figures, the pursuit of mathematical rigor, and the emergence of new mathematical topics (Palsun). The analysis revealed cases of recognizing figures as objects of inquiry, adding Western methods, replacing existing solutions, describing calculation validity, using diagrams, and a clear understanding of approximations.