IGCSE Maths / Graphs and transformations
Trig ratios of obtuse angles revision notes
Edexcel International GCSE Maths (4MA1), chapter 26: Graphs and transformations, specification point 4.8a. Part of the full course.
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For angles between 90° and 180°, sine stays positive, while cosine and tan become negative. These signs let you use trig ratios for obtuse angles and recognise when an angle found using the cosine rule is obtuse.