Purnama, P. W., and E. Retnowati. “The Effectiveness of Goal-Free Problems for Studying Triangle Similarity in Collaborative Groups”. JRAMathEdu (Journal of Research and Advances in Mathematics Education), vol. 6, no. 1, Dec. 2020, pp. 32-45, doi:10.23917/jramathedu.v6i1.11198.