<![CDATA[This development research aims to develop and assess the validity and effectiveness of teaching materials on solid geometry based on the Mandailing culture for use in the 5th grade of Elementary School. The research adopts a Research and Development approach, utilizing the ADDIE development model, which consists of five stages: Analysis, Design, Development, Implementation, and Evaluation. The instrument used in this study is a validation questionnaire. The research subjects consist of two validators, an instructional material expert and a Mathematics teacher. The validation results from the instructional material expert show a significant improvement before and after the revision of the teaching materials. Initially, the validity percentage of the teaching materials was 80% with a "Valid" criterion, equivalent to a score of 4. However, after the revision, the validity percentage increased to 93.75% with a "Highly Valid" criterion, equivalent to a score of 5. The validation results from the Mathematics teacher show a validity percentage of 98.75%, equivalent to a score of 5. Based on these assessments, it can be concluded that the developed solid geometry teaching materials based on the Mandailing culture meet the criteria of being "Highly Valid" for use in the teaching process. Thus, the findings of this study indicate that the teaching materials can be implemented in the 5th grade of Elementary School, and it is expected that these materials will enrich the learning process with local cultural values.]]>
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