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One of the goals of learning mathematics is that students must be able to understand mathematical concepts. At one high school in Kediri, class XI students did not understand mathematical concepts. The low ability to understand mathematical concepts is proven by the majority of students not being able to reformulate the solution to the problem given. Therefore, the aim of this research is to examine the ability to understand mathematical concepts of class XI students in solving problems related to trigonometry material. In this research, qualitative descriptive methods were used. Simple Random Sampling technique that researchers use to select research subjects. The students involved in the research were 23 students who were in class Furthermore, the non-test instrument takes the form of an unstructured interview to find out students' difficulties. The mathematical concept understanding ability rubric is used to analyze the mathematical concept understanding ability test. The research results show that the average understanding of the concept is categorized as sufficient at 64.50. Causal factors that also influence students' low percentage of understanding of mathematical concepts are lack of concentration in learning, irregular study habits and less interesting learning methods. Keywords: Concept Understanding, Mathematics, Trigonometry
Damayanti et al. (Wed,) studied this question.