Paper 2 · Trigonometry
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Lesson 6 of 8
Trigonometry 1.6: Cosine Rule
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- Find the third side from two sides and the included angle: a² = b² + c² − 2bc cos A
- Find any angle from three sides — rearrange to isolate cos A before substituting
- Read a negative cos A correctly: the angle is obtuse, not a mistake
- Choose between the sine rule and the cosine rule from the information given
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