Srichiangsa, W., and S. Art-in. “THE DEVELOPMENT OF ENGLISH SPEAKING ABILITY USING FLIPPED CLASSROOM WITH DUOLINGO FOR FIRST-YEAR VOCATIONAL CERTIFICATE STUDENTS”. Academic Journal Phranakhon Rajabhat University, vol. 17, no. 1, June 2026, pp. 47-61, https://so01.tci-thaijo.org/index.php/AJPU/article/view/285560.