TAF PUBLISHING

The purpose of this action research was to develop students' mathematical reasoning ability based on a constructivist approach. The topic used in this study was 'functions.' The participants were 35 Grade 10 students from a mathematics class at a secondary school in Bangkok, Thailand, studying in 2015. The research instruments were nine lesson plans on functions, a formative test, and a summative test focusing on mathematical reasoning. In each lesson plan, the researcher integrated a constructivist approach and reasoning. In this action research, there were two cycles of the action plan. Each cycle was composed of planning, doing, checking, and reflecting. The criteria for evaluating students' reasoning ability were at least 70% of all students to pass both formative and summative tests. To pass each test, each student had to get at least 70% of total score. Teaching and learning lasted for 13 periods with 50 minutes in each period. The researcher applied open-ended problems and cooperative learning to construct students' knowledge and reasoning ability (think-pair-share). Data were collected and analyzed from formative and summative tests, students' homework, worksheets, and cooperative learning (think-pair-share). This study showed that mathematical reasoning ability increased to the expected level required by the researcher. So, using the constructivist approach through open-ended problems and cooperative learning (think- pair- share) could increase students' reasoning ability.