Sumekar, W., F. Nurhasanah, and S. Sutopo. “Analyzing students’ Abstraction in Learning Common Tangent Lines of Two Circles Based on Cognitive Styles”. Beta: Jurnal Tadris Matematika, vol. 12, no. 2, Nov. 2019, pp. 94-108, doi:10.20414/betajtm.v12i2.325.