This paper aimed to expl ain how students construct the concept of the area of oblique triangles. Through Lesson Study using the Theory of Didactical Situations, the researchers designed a research lesson with the raison d'être of improvi ng the teaching of trigonometry. The research lesson was implemented to junior high school student-participants in a national high school in the Philippines, and was attended by a pool of observers. A priori and a posteriori analyses were conducted to validate the teaching and learning situations. Observations, notes, students’ outputs, and post-lesson discussions and reflections were used in the analysis of didactical situations. After the conduct of the Lesson Study, the following themes emerged: (1) explicit mathematics didactical intentions are essential in analyzing students' constructed mathematical knowledge and learning, and (2) balancing adidactical situation and teacher facilitation allows independent learning for students.