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Trigonometry is a necessary material for students to acquired. However, trigonometric continue to pose challenges for students, particularly cosine rule materials. The purpose of this study was thus to design learning series that assists students in grade tenth solve cosine rules problems using the Indonesian Realistic Mathematics Education context. The subject of the study is the tenth-grade students of SMA N 1 Belik, Pemalang, Central Java, with the involvement of 6 students who are selected as diverse students. The method followed design research using observation, video recordings activities in the online class, collecting student data, carrying out pre-test and post-test, and interviewing students as data collection techniques. This study produced a hypothetical learning trajectory (HLT) consisting of three activities in online class using Skype, namely: observing night market video to locate trigonometric ratios in a right triangle, observing takbir keliling video to discover the notion of the cosine rule, and resolve the contextual problems of the cosine rule. The study concluded that the activities designed can make hybrid learning meaningful and easy for students to grasp the cosine rule.

HLT; Cosine Rule; Hybrid Learning; Realistic Mathematics