STEM major students often enter colleges with a need to strengthen their foundational mathematics, such as trigonometry. We designed an innovative undergraduate course that aims to strengthen students' trigonometry knowledge application through programming mobile apps. We explored how students applied trigonometry concepts in developing their mobile apps and their overall experiences in this course. We found that the student-developed mobile apps included some combination of the following categories: quiz, game, and review guide for trigonometry learning, while trigonometry was applied in the mobile apps' codes and/or in the apps' content. Overall, students were very positive about their experiences of reviewing, revisiting, and utilizing trigonometry through programming mobile apps. The results suggest that the innovative curricular approach is promising in motivating students to apply foundational mathematics while solving design problems.