The purpose of this study was to determine if students’ mathematics selfefficacy could be increased and their mathematics anxiety could be reduced by adding supplemental mathematics practice using the internet-based website Khan Academy to increase mastery experiences in solving mathematics problems. The study focused on 156 Grades 10-12 students at Thai Christian School in Bangkok. A research experiment was conducted during the course of the second semester of the 2016 school year based on Bandura (1977) sources of self-efficacy ...