Akhmad Dzulfiqar Ilyas. “The Use of Duolingo Application to Improve Students’ Vocabulary Size at University Level”. Vidhyayana - An International Multidisciplinary Peer-Reviewed E-Journal - ISSN 2454-8596, vol. 8, no. 4, Feb. 2023, http://j.vidhyayanaejournal.org/index.php/journal/article/view/620.