• School Mathematics
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• v.12 no.3
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• pp.389-410
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• 2010

In this paper we examined the mutural relation of quadrilateral for the purpose to know the reason why we taught the mutural relation of quadrilateral in elementary school. We looked through the several materials, for example, national curriculum, textbooks, guide books for teachers in 1st, 2nd, 3rd curriculums. Finally we found that the mutural relation of quadrilateral was deeply involved in the concept of sets, or the concept of inclusion.