The lacking of studentsâ activities in learning mathematics and the less understanding of mathematical concept. The aims of this research are to 1) know  how the development of student activities is by applying the model of cooperative learning of club-round technique to the grade-IX students of SMPN 2 Lembang Jaya, 2) whether the mathematical concept understanding by applying such model is better than that by using the conventional learning to the grade-IX students of SMPN 2 Lembang Jaya. Population is the students grade-IX of SMPN 2 Lembang Jaya of academic 2012/2013 years. The sampling is made randomly and the chosen samples are grade-IX 3 as an experimental class and grade-IX 4 as a control class. The type of research is an experimental research with random to subject design. The analysis technique of the data using activities observasition and test t by one side. Based on the analysis of data, it is obtained that 1) the studentsâ activities experience an improvement by applying the model of Cooperative Learning of Club-Round Technique, 2) t result 2,02 > t tabel 1,69  So that, the studentsâ mathematical concept understanding applying such model is better than the studentsâ mathematical concept understanding using the conventional learning to the grade-IX students of SMPN 2 Lembang Jaya of the year 2012/2013.
                        
                        
                        
                        
                            
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