Ratnasari, Sinta, and Wahyu Setiawan. “ANALISIS KESULITAN BELAJAR SISWA PADA MATERI HIMPUNAN”. Journal on Education 1, no. 2 (February 28, 2019): 473-479. Accessed February 18, 2020. http://jonedu.org/index.php/joe/article/view/94.