Perwitasari, Dewi, and Edy Surya. 2017. “The Development of Learning Material Using Problem Based Learning to Improve Mathematical Communication Ability of Secondary School Students”. International Journal of Sciences: Basic and Applied Research (IJSBAR) 33 (3):200-207. https://www.gssrr.org/index.php/JournalOfBasicAndApplied/article/view/7382.