Saftari, M., D. Darmawijoyo, dan Y. Hartono. “Development of Student Activities Sheet Based on Apos Theory to Understand the Concept of Riemann Sum”. Math Didactic: Jurnal Pendidikan Matematika, Vol. 6, no. 1, Mei 2020, hlm. 110-23, doi:10.33654/math.v6i1.914.